AlllOb 13ab73

NK^IK /J-281

AFOSR SCIENTIFIC REPORT

AFOSR-TR-74-0063

Thermodynamics of Chemical Species

Important to Rocket Technology

Various authors

Physical Chemistry Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

1 July 1973

Final Report for Agreement No. AFOSR-ISSA-73-0001, Covering Period

January - June 1973

/oo 3d for ce Office of Scientific Research

ilson Boulevard

^•7? O P / Virginia 22209

NBSIR 73-281

AFOSR SCIENTIFIC REPORT

AFOSR-TR-74-0063

THERMODYNAMICS OF CHEMICAL SPECIES

IMPORTANT TO ROCKET TECHNOLOGY

Various authors

Physical Chemistry Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

1 July 1973

Final Report for Agreement No. AFOSR-1SSA-73~0001, Covering Period

January - June 1973

Prepared for Air Force Office of Scientific Research

1400 Wilson Boulevard

Arlington, Virginia 22209

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

FOREWORD

Structure, propulsion, and guidance of new or improved weapons

delivery systems are dependent in crucial areas of design on the

availability of accurate thermodynamic data. Data on high-temperature

materials, new rocket propellant ingredients, and combustion products

(including exhaust ions) are, in many cases, lacking or unreliable.

A broad integrated research program at the National Bureau of

Standards has supplied new or more reliable thermodynamic properties

essential in several major phases of current propulsion development

and application. Measured were compounds of those several chemical

elements important in efficient propulsion fuels; those substances

most affecting ion concentrations in such advanced propulsion concepts

as ion propulsion; and the transition and other refractory metals (and

their pertinent compounds) which may be suitable as construction materials

for rocket motors, rocket nozzles, and nose cones that will be durable

under extreme conditions of high temperature and corrosive environment.

The properties determined extend in temperature up to 6000 degrees

Kelvin. The principal research activities were experimental, and

involved developing new measurement techniques and apparatus, as well

as measuring heats of reaction, of fusion, and of vaporization; specific

heats; equilibria involving gases; several properties from fast processes

at very high temperatures; spectra of the infrared, matrix-isolation,

microwave, and electronic types; and mass spectra. Some of these techniques,

by relating thermodynamic properties to molecular or crystal structures,

make it possible to tabulate reliably these properties over far wider

ranges of temperature and pressure than those actually employed in the

basic investigations. Additional research activities of the program have

involved the critical review of published chemical -thermodynamic (and some chemical-kinetic) data, and the generation of new thermochemical

tables important in current chemical-laser research.

1
8504 8628 8919 9028' 93Sq' lien' Itl'.' ^^^S, 8186,

10326, 10481, 10904 'fid TMsiR'73 280 '

'

ii
ABSTRACT

This report (covering January-June 1973) constitutes the Final

Report on all past phases of the program excep'; that measuring

thermophysical properties by high-speed techniques, which is continuing.

The nature and scope of the subjects covered in this report may

be inferred from the detailed Table of Contents to be found on the

following pages.

Using a subsecond -duration transient technique, the specific

heat, electrical resistivity, and hemispherical total emittance were

simultaneously measured over the temperature range 1500-2800 K for

iron (99.9 + 7. pure) and for the alloy 80 Nb-10 Ta-10 W (nominal

composition by weight), with estimated uncertainties of 3%, 0.5-1%,

and 37o^ respectively. The specific heats found for the alloy are about 2.57c. above the additive (Kopp ' s law) values; the electrical

resistivity shows significant departure f rom Matthiesen ' s law, the

resistivity-temperature curve showing (like pure Nb) negative departure

from linearity. The specific heat and electrical resistivity of iron

both increase during the gamma-to-delta transformation (1673 and

1683 K on two specimens, with an estimated heat of transformation of

890 J mol" ) . The work on iron demonstrates the feasibility of

applying a rapid-pulse heating technique to solid-solid transformations.

The following melting points (and normal spectral emittance for 650 nm at the melting point) were found: Fe, 1808 ±5K (0.355); 99Nb-l Zr,

2737 ±10 K (0.351); 80 Nb-10 Ta- 10 W, 2814 ±10 K (0.333); 90 Ta-10 W,

3286 ±15 K (0.396)

.

Preliminary values are reported for the first time for the standard

heat of formation and a vapor pressure of the subfluoride of molybdenum

M0F5. Solution calorimetry, involving a series of steps including

oxidation by Xe03 in aqueous HF, yielded, at 298 K,Z^f [MoF3(c) ] = -1395.74 ± 4.6 kj mol"^ (-333.59 ± 1.1 kcal mol'l), with a maximum

uncertainty of about 17 kJ mol"-*- (4 kcal mol"-*-) in AH^ [MoFg(^) ] - AHj [MoF5(c)]; final results will include some minor heats of dilution.

(Earlier solution calorimetry involving alkaline solutions of XeO^ was unsatisfactory owing to spontaneous decomposition of the XeO^.) A new

static method for measuring small vapor pressures has reproduced known

values for I2 and MoFg to within 17c,. In extending the method to MoF^,

it was found that the MoFg (added to suppress sample disproportionation)

could be accounted for reliably only after crystallizing the M0F3 . The

vapor pressure of MoF^ (£) thus measured at 393 K, when compared with

the earlier transpiration result for this temperature, indicates that

the saturated vapor contains about 85 or 90 mole 7, of the monomer species,

depending on the method of interpretation. .on. J

Thomas B. Douglas J ^

Charles W. Beckett

iii

TABLE OF CONTENTS

Page

Foreword i

Abstract iii

Chap. 1. SIMULTANEOUS MEASUREMENTS OF SPECIFIC HEAT,

ELECTRICAL RESISTIVITY AND HEMISPHERICAL

TOTAL EMITTANCE OF THE TERNARY ALLOY NIOBIUM

(80) - TANTALUM ( 10) -TUNGSTEN (10) IN THE

RANGE 1500 TO 2800 K

(by Ared Cezairliyan) 1

Abstract 1 1. Introduction 2

2. Measurements 2

Specimen 2

Procedure 3

3. Experimental Results 4

Specific Heat 4

Table 1. Specific heat,

electrical resistivity,

and hemispherical total

emittance of the ternary

alloy Nb(80)-Ta(10)-W(10) 5

Fig. 1. Deviation of specific

heat results from

equation (1) 6

Electrical Resistivity 7 Hemispherical Total Emittance .... 7 Fig. 2. Deviation of electrical

resistivity results from

equation (2) 8

Estimate of Errors 9

4. Discussion 9

Fig. 3. Specific heat of the

ternary alloy 80Nb-10TalOW and the constituent

elements 10

Fig. 4. Electrical resistivity

of the ternary alloy

80Nb-10Ta-10W and the

constituent elements . . 11

Fig. 5. Hemispherical total

emittance of the

ternary alloy 80Nb-10TalOW and the constituent

elements 12

Fig. 6. Departure of the specific

heat of the ternary alloy

80Nb-10Ta-10W from Kopp '

s

law 14

v
TABLE OF CONTENTS (Continued)

Page

i. Fig. 7. Variation of the

quantity

J-'q, defined

by equation (6), as a function of temperature

for the ternary alloy

80Nb-10Ta-10W 16

Acknowledgement 17

5. Appendix 18

Table A-1. Experimental results on specific heat and

electrical resistivity

of the ternary alloy

Nb(80)-Ta(10)-W(10) ... 18

Table A-2. Experimental results on hemispherical total

emittance of the ternary

alloy Nb(80)-Ta(10)-W(10) 19

6. References 20

Chap. 2. THERMOPHYSICAL MEASUREMENTS ON IRON ABOVE

1500 K USING A TRANSIENT (SUBSECOND) TECHNIQUE

(by A. Cezairliyan and J. L. McClure) 22

Abstract 22

1. Introduction 23

2. Measurements 23

Specimens 23

Procedure 24

3. Experimental Results 24

Table 1. Heat capacity and

electrical resistivity

of iron 25

Heat Capacity 26

Electrical Resistivity 26

Hemispherical Total Emittance 26

Fig. 1. Variation of temperature

of iron (specimen-2) as

a function of time near

and at the melting

, point 27

Melting Point 28

Estimate of Errors 28

4. Discussion 28

Fig. 2. Variation of temperatures

of two iron specimens

as a function of time at

the melting point ... 29

Fig. 3. Heat capacity of iron

i( reported in the literature 31

Fig. 4. Electrical resistivity

of iron reported in the

literature 32

Table 2. Melting point of pure iron 33

vi
TABLE OF CONTENTS (Continued)

Page

Appendix 34

Table A-1. Experimental results

on heat capacity and

electrical resistivity

of iron 34

5. References 35

Chap. 3. A SUBSECOND PULSE HEATING TECHNIQUE FOR

THE STUDY OF SOLID -SOLID PHASE TRANSFORMATIONS

AT HIGH TEMPERATURES: APPLICATION TO IRON

(by A. Cezairliyan and J. L. McClure) 37

Abstract 37

1. Introduction 38

2. Method . 38

3. Measurements 39

Table 1. Impurities in the iron

specimens 40

Table 2. Summary of the type

and dimensions of the

iron specimen 41

Fig. 1. Oscilloscope trace

photograph showing

radiance of a rodshape specimen during

heating 42

Fig. 2. Heating curves showing

solid-solid phase

transformation (v to

)) in iron for a tubesnape shap SIspecimen

Fig. 3. Heating curve showing

solid-solid phase

transformation (v to

6) in iron for a rod

specimen 45

4. Results 46

Fig. 4. Heating curves for

tube-shape specimens

of Ferrovac E iron

during the v to 6 transformation 47

Fig. 5. Heating curves for the

tube-shape specimens

of SRM 734 iron during

V to 6 transformation . 48

vii
TABLE OF CONTENTS (Continued)

Page

Fig. 6. Heating carves for

rod-shape specimens

of Ferrovac E iron

during Y to 5 transformation 49

Fig. 7. Heating curves for rodshape specimens of

SRM 734 iron during V

to 6 transformation ... 50

Table 3. Average transformation

temperature and heating

rate just before

transformation for

tube-shape iron

specimens 51

Table 4. Average radiance temperature at V to 6 transformation point,

heating rate just before

transformation, and rad

.

temp. at melting point

for rod-shaped iron

specimens 51

Fig. 8. Radiance temperatures,

melting points, and

temperatures at iron

transformation point . . 52

Table 5. Normal spectral

emittance of iron at

V to 6 transformation

and melting points ... 54

Table 6. Temperature and energy

of V to 6 transformation

in iron (literature) . . 54

5. Discussion 55

Acknowledgement 57

6. References 58

Chap. 4. MEASUREMENT OF MELTING POINT, NORMAL SPECTRAL

EMITTANCE (AT MELTING POINT) AND ELECTRICAL

RESISTIVITY (NEAR MELTING POINT) OF SOME

REFRACTORY ALLOYS

(by Ared Cezairliyan) 60

Abstract 60

1. Introduction 61

2. Measurements 61

Table 1. Composition of the

alloys 62

viii
TABLE OF CONTENTS (Continued)

Page

Table 2. Specimen designations

and the results of

individual experiments . 63

2.1. Melting Point 64

2.2. Normal Spectral Emittance .... 64

Table 3. Melting point, and

radiance temperature

and normal spectral

emittance at melting

point, of some refractory

alloys 65

Fig. 1. Variation of temperature

of various refractory

alloys as function of

time at respective

melting points 66

2.3. Electrical Resistivity .... 67

Fig. 2. Variation of radiance

temperature of 90Ta-10W

and 99Nb-lZr as function

of time near and at

respective melting points 68

Fig. 3. Variation of radiance

temperature of 80Nb-10

Ta-lOW as function of

time near and at melting

point 69

2.4. Estimate of Errors 70

Table 4. Electrical resistivity

of some refractory alloys

near their melting points 71

Fig. 4. Electrical resistivity

of 90Ta-10W near and at its

melting point 72

Fig. 5. Electrical resistivity

of 99Nb-lZr near and at its

melting point 73

Fig. 6. Electrical resistivity of

BONb-lOTa-lOW near and

at its melting point . . 74

3. Discussion 75

4. References 77

ix
TABLE OF CONTENTS (Continued)

Chap. 5. THE ENTHALPY OF FORMATION OF MoF ^ Cc) BY

SOLUTION CALORIMETRY: PRELIMINARY ANALYSIS

OF EXPERIMENTS Page

(by R. L. Nuttall, M. V. Kilday, and

K. L. Churney) 78

1. Introduction 78

2. Reaction Schemes of Solution Experiments . 79

3. Experimental 81

a . Materials 81

b. Calorimeter 81

4. Results 81

a. Experimental Results 81

b. Evaluation of (m^-m-^) 83

c. Evaluation of 83

d. Evaluation of the enthalpy of

formation of M0F3, AH°, [MoF^] . . 84

5. References 85

6. Appendix--Preliminary Experiments .... 86

Table 1. Calorimetric Data for

Reaction (1) 88

Table 2. Calorimetric Data for

Reaction (2) 89

Chap. 6. THE VAPOR PRESSURE AT 393 K OF LIQUID

MOLYBDENUM PENTAFLUORIDE, M0F 3. ACCORDING

TO PRELIMINARY STATIC -METHOD MEASUREMENTS

(By Thomas B. Douglas and Ralph F. Krause, Jr.) . . 90

Abstract 90

I. Introduction 90

II. Tests of the New Apparatus by Measuring

the Vapor Pressure of MoFg 94

Table 1. Test of the StaticMethod: Vapor

Pressure of MoFg(s)

at 236.60 K 96

III. Preliminary: Static-Method Vapor-Pressure

Measurements on KoF^(Jl) at 392.6 K 98

Fig. 1. Revised static-method

apparatus used to

measure the vapor

pressure of M0F3

(schematic) 103

Table 2. Static-Method Measurements of the Vapor

Pressure of MoFr(^) at

392.6 K 106

IV. References 110

X
Chapter 1 SIMULTANEOUS MEASUREMENTS OF SPECIFIC HEAT,

ELECTRICAL RESISTIVITY AND HEMISPHERICAL

TOTAL EMITTANCE OF THE TERNARY ALLOY

NIOBIUM (80) - TANTALUM (10) - TUNGSTEN (10)

IN THE RANGE 1500 TO 2800 K''

Ared Cezairliyan

National Bureau of Standards

Washington, D. C. 20234

Abstract

Simultaneous measurements of the specific heat, electrical

resistivity, and hemispherical total emittance of the ternary alloy

niobium-tantalum-tungsten (nominal composition, 80-10-10 percent

by weight) in the temperature range 1500 to 2800 K by a subsecondduration, transient technique are described. Estimated inaccuracies

of measured properties are: 3% for specific heat and hemispherical

total emittance, and 0.57o for electrical resistivity. Properties

of the alloy are compared with the properties of the constituent

elements. The values of measured specific heat are approximately

2.6% (on the average) higher than the values computed according

to Kopp ' s additivity law. However, this difference is within the

combined estimated errors. The electrical resistivity results

indicate a significant departure from Matthiessen ' s law. Like the

major constituent niobium, the alloy showed a negative departure from

linearity in the curve of electrical resistivity versus temperature.

This work was supported in part by the U. S. Air Force Office of

Scientific Research.

1
1. Introduction

In this paper, application of a transient technique to the

simultaneous measurements of specific heat, electrical resistivity,

.and hemispherical total emittance of the ternary alloy niobiumtantalum- tungs ten (nominal composition 80-10-10 percent by weight)

in the temperature range from 1500 to 2800 K is described.

The method is based on rapid resistive self-heating of the

specimen from room temperature to high temperatures (above 1500 K)

in less than one second by the passage of an electrical current

pulse through it; and on measuring, with millisecond resolution, such

experimental quantities as current through the specimen, potential

drop across the specimen, and specimen temperature. Details regarding

the construction and operation of the measurement system, the

methods of measuring experimental quantities, and other pertinent

information, such as the formulation of relations for properties,

etc. are given In earlier publications [1, 2]^.

2. Measurements

2 Specimen The specimen was a tube fabricated from a rod

by removing the center portion by an electro-erosion technique.

The nominal dimensions of the specimen were: length, 102 mm; outside

diameter, 6.3 mm; and wall thickness, 0.5 mm. The outer surface

of the specimen was polished to reduce heat loss due to thermal

radiation,

''"Figures in brackets indicate the literature references at the end

of this paper,

2The specimen in rod form was furnished by the Fansteel Metals Corporation.

2
According to the manufacturer's analysis, the composition

(by weight) of the major constituents of the specimen was: niobium,

81%; tantalum, 9.7%; and tungsten, 9.3%. The impurities (in ppm)

were: hafnium, molybdenum, silicon and zirconium, 100 each; iron

and titanium, 50 each; C, 11; 0, 51; N, 44; and H, 5.

Procedure : The measurements were performed in the temperature

interval 1500 to 2800 K. To optimize the operation of the pyrometer,

this interval was divided into six ranges. One experiment was

performed in each range. Before the start of the experiments, the

specimen was heat treated by subjecting it to 30 heating pulses

(up to approximately 2200 K) . All the experiments were conducted

with the specimen in a vacuum environment of approximately 10

^

torr. To optimize the operation of the measurement system, the

heating rate of the specimens was varied depending on the desired

temperature range by adjusting the value of a resistance in series

with the specimen. Duration of current pulses in the experiments

ranged from 430 to 500 ms ; and the heating rate ranged from 3500

to 5000 K s . Radiative heat loss from the specimen amounted to

approximately 1% at 1500 K, and 14% at 2800 K of the input power.

3
3. Experimental Results

The properties reported in this paper are based on the

International Practical Temperature Scale of 1968 [3]. In all

.computations^ the geometrical quantities are based on their room

temperature (298 K) dimensions. The experimental results for

properties are represented by polynomial functions in temperature

obtained by least squares approximation of the individual points.

The final values of properties at 100 degree temperature intervals

computed using the functions are presented in Table 1. Results

obtained from individual experiments are given in the Appendix

(Tables A-1 and A-2) . Each number tabulated in these tables

represents results from over fifty original data points.

Specific Heat. Specific heat was computed from data taken

during the heating period. A correction for power loss due to

thermal radiation was made using the results on hemispherical total

emittance. The function for specific heat (standard deviation =

0.47o) that represents the results in the temperature range 1500 to

2800 K is:

c = -0.1042 + 5.889 x 10"S-3.019 x 10"'^T^ + 5.758 x 10"-'--'"T^ (1)

P

where T is in K, and c is in J g ''"K The deviations of the > p

6

experimental results from equation (1) are shown on figure (1).

4
Table 1

Specific heat, electrical resistivity, and hemispherical total

emittance of the ternary alloy niobium (80) - tantalum (10)- tungsten (10)

llemp

.

i K

-1 -1 10 Q. m

a

e

1500 0.2942 57.27

i

i

i 1600 0.3010 60.02 i

i 1700 0.3073 62.74 0.217

1 1800 0.3135 65.43 0.227 1

11900

i

0.3198 68.10

i 0.237 \

;2000

1 0.3266 70.74

j 0.246 1

i2100 0.3344 73 .36 0.255 j

i i|2200 0.3433 75 .95 0.264

|2300 0.3538 78 .51 0.272

2400 0.3662 81.05 0.280

2500 0.3809 83.55 0.287

2600 0.3981 86.04 0.294

2700 0.4183 88.49

2800 1 0.4418 90.92

Based on ambient temperature (298 K) dimensions.

5
O

O lO

O

—

O o -r lN30d3d *N0llVIA3a 6
Electrical Resistivity : The electrical resistivity was

determined from the same experiments that were used to calculate

the specific heat. The function for electrical resistivity (standard

deviation = 0.03%) that represents the results in the temperature

range 1500 to 2800 K is:

P = 12.86 + 3.160 X 10"^T-1.329 x 10"^T^ (2)

- 8 where T is in and D is in 10 O m. The deviations of the

experimental results from equation (2) are shown on figure 2. The

measurement, before the pulse experiments, of the electrical

resistivity of the specimen at 293 K with a Kelvin bridge yielded

_g

a value of 17.2 x 10 Q m.

Hemispherical Total Emittance : Hemispherical total emittance

was computed using data taken during both heating and initial free

radiative cooling periods. The function for hemispherical total

emittance (standard deviation = 0.6%) that represents the results

in the temperature range 1700 to 2600 K is:

e = -1.055 X 10"^ + 1.657 x lO'S-1.864 x 10"^T^ (3)

where T is in K.

7
8
Estimate of Errors: The details for estimating errors in

measured and computed quantities in transient experiments using

the present measurement system are given in an earlier publication

[2]. In this paper, the specific items in the error analysis were

recomputed whenever the present conditions differed from those in

3

the earlier publication. The results for imprecision and

4 inaccuracy in the properties are: 0.4% and 3% for specific heat,

0.037o and 0.57o for electrical resistivity, 0.6% and 3% for

hemispherical total emittance.

4. Discussion

The specific heat, electrical resistivity and hemispherical

total emittance of the ternary alloy niobium (80) - tantalum (10) -

tungsten (10) alloy measured in this work are presented graphically

in figures 3, 4, and 5, respectively. For comparison purposes,

similar results for niobium, tantalum and tungsten metals, measured

and reported earlier [4, 5, 6] are also included in the figures.

Imprecision refers to the standard deviation of an individual point

as computed from the difference between measured value and that from

the smooth function obtained by the least squares method.

4

Inaccuracy refers to the estimated total error (random and systematic)

.

9
10

30NV11IIAI3 nVlOl nV0ld3dSIIAI3H

12
According to Kopp ' s law, specific heat of a ternary alloy,

c^23 (o'^ mass basis), may be expressed as

c^23 = ^i^i + X2C2 + x^c^

where x^, X2 and x^ are mass fractions of the constituent elements

1, 2 and 3, and c^, C2 and c^ are their respective specific heats.

Departure of the measured specific heat of this work from Kopp

law was computed using the data on niobium, tantalum and tungsten

reported in earlier publications [4, 5, 6].

The results are presented graphically in figure 6. It may

be seen that in the temperature interval 2000 to 2800 K (interval

in which data on the three constituent elements are available) the

measured values are approximately 2.67o (on the average) higher than

the computed values. The results of earlier measurements [7] of

the specific heat of the binary alloy tantalum (90) - tungsten (10)

have also shown a similar trend, that is, the measured values of

the alloy were approximately 2% higher than the computed values

according to the Kopp ' s law. However, too much significance should

not be placed to the difference between measured and computed

specific heat values since its magnitude is less than the combined

estimated errors in the measurements of specific heat of the alloy

and its constituents.

13
%'33N3d3JJia 1V3H 3UI33dS
The Matthiessen ' s law for the electrical resistivity of a

ternary alloy, ^122' expressed as

Pl23 = ^iPl ^ ^2^2 + ^3^3 + Po

'where X^, X2 and are atomic fractions of the constituent elements

1, 2 and 3, and D^, and are their respective electrical

resistivities. The quantity p^ is considered to be constant (temperature independent) for a given alloy.

Rearrangement of Equation (5) yields

Po = P123 - ^^1^1 + ^2P2 + ^3P3>

Departure of the electrical resistivity of the ternary alloy

of this work from Matthiessen' s law is determined using Equation (6).

The measured values for the alloy are substituted for P-[^23 ^'^'^ data

on niobium, tantalum and tungsten reported in earlier publications

[4, 5, 6] are used for p^, and p^ . The results are presented

graphically in figure 7 in terms of the quantity p^. According

to the law, p^ should be constant, however computations show that

it decreases with increasing temperature and undergoes sign reversal

at high temperatures . The two curves in figure 7 were obtained by

fitting the high temperature points to linear and quadratic functions

using the least squares method, and forcing the functions to pass

through the room temperature value. Average absolute difference

_ g of the points from the linear and quadratic functions is 0.02 x 10

-8

O m and 0.1 x 10 0 m, respectively. Since either one of these values

15

is considerably less than the estimated uncertainty of the individual

points (1.1 X 10 0 which is obtained from the combination of

estimated errors in the electrical resistivity of the alloy and

its three constituents) the form of the function (linear or quadratic)

is not significant. However, the general trend, that is decrease

in the quantity with increasing temperature, is significant.

_ g At 293 K, electrical resistivity of the alloy (17.2 x 10 0 m

is higher than the resistivity of any one of its constituents

_g _g

(15.9 X 10 O m for niobium, 14.0 x 10 o m for tantalum and

-8

5.45 X 10 O m for tungsten). However, at high temperatures (figure

4) the resistivity of the alloy is lower than that of the major

constituent, niobium. A similar trend was also observed in the

electrical resistivity of the binary alloy tantalum (90) - tungsten

(10) reported earlier [7]. Like niobium, at high temperatures the

alloy showed a negative departure from linearity in the curve of

electrical resistivity versus temperature.

Acknowledgement

The author expresses his gratitude to Dr. C. W. Beckett for

his encouragement of research in high-speed thermophysical measurements and to Mr. M. S. Morse for his help with the electronic

instrumentation

.

17
5. Appendix

Table A-1

Experimental results on Sr/ecific ".leat and

electrical resistivity of the ternary alloy

niobium (80) - tantalum (10) - tungsten (10) T

(K) (J g K )

P

(7o)

P /in ~^r\ TM^ (iU n m)

A p^

(/o)

1500 0 .2947 +0.18 57.25 -0.03

1550 0 .2972 -0.17 58.64 -0.01

1600 0 .3011 +0.04 60.02 +0.01

1650 0 .3064 +0.73 61.39 +0.02

1700 0 .3058 -0.50 62.75 +0.03

1750 0 .3098 -0.18 64.10 +0.02

1800 0 .3138 +0.11 65.46 +0.05

1850 0 .3134 -1.01 66.76 -0.01

1900 0 .3187 -0.35 68.09 -0.02

1950 0 .3240 +0.26 69.41 -0.02

2000 0 .3289 +0.69 70.74 0

2050 0 .3299 -0.14 72.04 -0.02

2100 0 .3342 -0.05 73.34 -0.02

2150 0 .3391 +0.12 74.64 -0.03

2200 0 .3446 +0.37 75.93 -0.03

2250 0 .3509 +0.72 77.21 -0.03

2300 0 .3535 -0.10 78.54 +0.04

2350 0 3594 -0.10 79.80 +0,03

2400 0 3658 -0.13 81.06 +0.01

2450 0 3725 -0.20 82.30 0 2500 0 3798 -0.29 83.54 -0.02

2550 0 3877 -0.39 84.7 7 -0.03

2600 0 3983 +0.03 86.09 +0,06

2650 0 4082 +0.08 87.30 +0.03

2700 0 4187 +0.08 88.51 +0,02

2750 0 4300 +0.07 89.71 0

2800 0 4422 +0.07 90.87 -0,06

The quantities A Cp and A p are percentage deviations of the individual

results from the smooth functions represented by equations (1) and (2),

respectively

,

18
Table A-2

Experimental results on hemispherical total emittance

of the ternary alloy niobium (80) - tantalum (10) - tungsten (10)

T e

CK) a)

1693.2 0.2171 +0.23

i 1693.6

1 0.2158 -0.39

1

i 1693.7 0.2186 +0.89

1694.1 0.2180 +0.60

1

i 1885.0 0.2329 -1.15

1885.6 0.2326

! -1.31

1 1885.6 0.2356 -0.02 I

1 1886.2 0.2360 +0 . 13

j 2053 .0 0.2512 +0.04 i

2053.9 0.2506 -0.23 !

2053.9 0.2519 +0.29

2054.8 0.2516 +0.14

2333 .2 0.2748 +0.07

2334.7 0.2753 +0.21

2334.7 0.2757 +0,35

2336.2 0.2764 +0.56

2663.2 0.2976 -0.32

2665.6 0.2981 -0.21

2665.6 0.2986 -0.04

2668.0 0.2993 +0.14

The quantity Ac is percentage deviation of the individual results

from the smooth function represented by equation (3)

.

19
6. References

Cezalrliyan, A., Design and operational characteristics of

a high-speed (millisecond) system for the measurement of

themophysical properties at high temperatures, J. Res. Nat.

Bur. Stand. (U. S.), 75C (Eng. and Instr.), 7 (1971).

Cezairliyan, A., Morse, M. S., Berman, H. A., and Beckett, C. W.

High-speed (subsecond) measurement of heat capacity, electrical

resistivity, and thermal radiation properties of molybdenum

in the range 1900 to 2800 K, J. Res. Nat. Bur. Stand.

(U. S.), 74A (Phys. and Chem.), 65 (1970).

International Practical Temperature Scale of 1968, Metrologia,

5, 35 (1969).

Cezairliyan, A., High-speed (subsecond) measurement of heat

capacity, electrical resistivity, and thermal radiation

properties of niobium in the range 1500 to 2700 K, J. Res.

Nat. Bur. Stand. (U. S.), 75A (Phys. and Chem.), 565 (1971).

Cezairliyan, A., McClure, J. L., and Beckett, C. W., Highspeed (subsecond) measurement of heat capacity, electrical

resistivity, and thermal radiation properties of tantalum

in the range 1900 to 3200 K, J. Res. Nat. Bur. Stand. (U. S.),

75A (Phys. and Chem.), 1 (1971).

20
Cezairliyan, A., and McClure, J. L., High-speed (subsecond)

measurement of heat capacity, electrical resistivity, and

thermal radiation properties of tungsten in the range 2000

to 3600 K, J. Res. Nat. Bur. Stand. (U. S.), 75A (Phys. and

Chem.), 283 (1971).

Cezairliyan, A., High-speed (subsecond) simultaneous measurement of specific heat, electrical resistivity, and hemispherical

total emittance of tantalum-10 (wt . 7o) tungsten alloy in the

range 1500 to 3200 K, High Temperatures -High Pressures, 4- » 541 (id7Z).

21
Chapter 2 THERMOPHYSICAL MEASUREMENTS ON IRON AF.OVE 1500 K

USING A TRANSIENT (SUBSECOND) TECHNIQUE

A. Cezairliyan and J. L. McClure

Institute for Materials Research

National Bureau of Standards

Washington, DC 20234

Abstract

Simultaneous measurements of heat capacity, electrical resistivity

and hemispherical total emittance of iron (99.9"*'% pure) in the temperature range 1500 to 1800 K, and the melting point of iron by a subsecond

duration, transient technique are described. The measurements indicate

increases in heat capacity and electrical resistivity as the result of

the solid-solid phase transformation in iron. The measured value

of the hemispherical total emittance at 1720 K is 0.33. The average of

the results of two experiments yield a value of 1808 K for the melting

point of iron. Estimated inaccuracies of measured properties are: 3%

for heat capacity and emittance, 1% for electrical resistivity, and 5 K

for the melting point.

22
1. Introduction

Most of the reliable measurements of thermophysical and related

properties of iron reported in the literature were performed at temperatures below 1500 K. Even at these temperatures, considerable disagreements exist between the results of various investigations. To a large

extent, these disagreements may be attributed to difficulties associated

with the magnetic and allotropic transformations in iron. The objective

of the present work is to measure selected properties (heat capacity,

electrical resistivity, hemispherical total emittance and melting point)

of iron above 1500 K using a transient technique.

The method is based on rapid resistive self-heating of the specimen from room temperature to high temperatures in less than one second

by the passage of an electrical current pulse through it; and on measuring with millisecond resolution, such experimental quantities as

current through the specimen, potential drop across the specimen, and

specimen temperature. Details regarding the construction and operation

of the measurement system, the formulation of relations for the properties, the methods of measuring experimental quantities, and other pertinent information are given in earlier publications [1,2]'''.

2. Measurements

2 Specimens : The specimens were tubes fabricated from rods by

removing the center portion using an electro-erosion technique. The

'Figures in brackets indicate the literature references at the end of

this paper.

2The specimens in rod form were furnished by the Office of Standard

Reference Materials (OSRM) of NBS (Standard Reference Material 734-S,

Electrolytic Iron, 99. 9"*"% pure). The details regarding the impurities

are documented by OSRM.

23
nominal dimensions of the specimens were: length, 102 mm; outside

diameter, 6.3 mm; and wall thickness, 0.5 mm. The outer surface of the

specimens were polished to reduce heat loss due to thermal radiation.

Procedure : Because of the existence of a solid-solid phase transformation (y^<5) in iron at approximately 1682 K, the measurements were

performed in the following two ranges: low (1500-1660 K) , and high

(1700-1800 K) . Four experiments were conducted on two specimens; two

experiments in the low range and two experiments in the high range.

The duration of the current pulses in the experiments ranged from 500

to 900 ms, and the heating rate ranged from 2000 to 3500 K s

Radiative heat loss from the specimen was, in all cases, less than 2%

at 1500 K and less than 4% at 1800 K of the input power. All the

experiments were conducted with the specimen in a vacuum environment

of approximately 10 ^ torr.

3. Experimental Results

The properties reported in this paper are based on the International

Practical Temperature Scale of 1968 [3]. In all computations, the geometrical quantities are based on their room temperature (298 K) dimensions. The experimental results for the heat capacity and the electrical

resistivity of iron (for each phase) are presented as linear functions

in temperature. These functions were obtained by least squares approximation of the individual points corresponding to the measurements on the

two specimens. The final values of the properties at 20 degree temperature intervals computed from these functions are presented in table 1.

The results of individual experiments are given in the Appendix.

24
TABLE 1

Heat capacity and electrical resistivity of iron

Temp. c P

P -1 -1 -8

K J mol K 10 n m

1500 36.12 120.66

- 1520 36.37 121.10

1540 36.61 121.54

1560 36.86 121.98

Y-iron 1580 37.11 122.42

1600 37.35 122.86

1620 37 . 60 123.29

1640 37.84 123.73

1660 38.09 124.17

1700 41.46 125.35

1720 42.09 125.65

1740 42.72 125.96

6-iron

1760 43.35 126.26

1780 43.99 126.57

1800 44.62 126.87

25
Heat Capacity : Heat capacity was computed from data taken during

the heating period. A correction for power loss due to thermal radiation

was made using the single measured value of hemispherical total emittance

in conjunction with the slope of emittance versus temperature function

obtained from the literature [4].

The functions for heat capacity are:

In the temperature range 1500 to 1660 K (y-iron)

,

c = 17.64 + 1.232 X lO"^ T (1)

P

In the temperature range 1700 to 1800 K (6-iron),

Cp = -12.28 + 3.161 X 10"^ T (2)

where T is in K, and c is in J mol K The standard deviation for

P

equations (1) and (2) is 0.4%. In the computations of heat capacity,

the atomic weight of iron was taken as 55.85.

Electrical Resistivity ; The electrical resistivity was determined

from the same experiments that were used to calculate the heat capacity,

The functions for electrical resistivity are:

In the temperature range 1550 to 1660 K (y-iron)

,

P = 87.72 + 2.196 x lO"^ T (3)

In the temperature range 1700 to 1800 K (6-iron),

p = 99.37 + 1.528 x lO"^ T (4)

—8

where T is in K and p is in 10 0 m. The standard deviation for equations

(3) and (4) is 0.5%. The electrical resistivity of one of the specimens

—8

measured at 293 K using a Kelvin bridge was 10.2 x 10 0 m.

Hemispherical Total Emittance : In one of the experiments in the

lower temperature range data were taken during the initial cooling period

r

26
>-<

en

cr<

LU

Ic

o

•H

4-1 o

c

IC

0)o

0)fxM

o

1-1 •Hm

o

u

•P

CO

t-t (U

14-1 o

c

o

•H

4-1 •H

)-4 >

Pi:; !=> O

M

B

00

O

P.C

•H

e

0)

4-1

IdC

C

0)e

>l*3dniVd3dlAI31

27
of the specimen which followed the heating period. The results yielded

a value of 0.33 for the hemispherical total emittance at 1720 K.

Melting Point : The melting point of a specimen is manifested by

a plateau in the temperature versus time function (figure 1). The

melting point of the specimen was determined by averaging the temperature

points at the plateau. The results for two specimens (figure 2) yielded

the values 1807.5 K and 1808.6 K, with standard deviations of 0.2 K and

0.4 K, respectively. It may be concluded that the melting point of iron

is 1808 K.

Estimate of Errors : The details for estimating errors in measured

and computed quantities using the present measurement system are given

in an earlier publication [2], In this paper, the specific items were

recomputed whenever the present conditions differed from those in the

earlier publication.

3 4 The results for imprecision and inaccuracy in the properties

are: 0.6% and 3% for heat capacity, 0.5% and 1% for electrical resistivity, 0.5 K and 5 K for the melting point. The inaccuracy in the

hemispherical total emittance is estimated to be 3%.

4. Discussion

The heat capacity and electrical resistivity results of this work

are compared graphically with those in the literature in figures 3 and

4, respectively.

3 Imprecision refers to the standard deviation of an individual point as computed from the difference between measured value and that from the

smooth function obtained by the least squares method.

4

Inaccuracy refers to the estimated total error (random and systematic).

28
LO

ino oUOc3 — CO I I

LU Ld

O

cr<

crm<

LU

ID

COcSmCa

(Ucuuoow

(U3

4-1 rt

>-i 0)e

(U

4-) > e

00e

•H

4J

4-1 c

•HO

(30 •H

4-t fH

0)0

OJ Xi

4-1

4J

CO

(U6

•H

4-1 o o

UJ LU

Q_ CL

CO CO

eg8H o o

00

CDo

CX)

O

00

>^'3dniVd3dl/\l31

29
From figure 3 it may be seen that at 1300 K considerable disagreements (up to approximately 10%) exist in the heat capacity values reported

in the literature; however, immediately below the y 6 transformation the

disagreements (especially between the results of recent investigations)

are reduced to a value less than 3%. In contrast to the constant values

for the heat capacity of 6-iron reported in the literature, the results

of the present work show that heat capacity increases with temperature

as the melting point is approached.

Figures 3 and 4 show that the heat capacity and electrical resistivity are discontinuous at the transformation point. Extrapolation of

the properties to the transformation temperature (1682 K, based on preliminary measurements) indicates increases of approximately 7% in the

heat capacity and 0.3% in the electrical resistivity during the y

6

transformation. The actual increase in the electrical resistivity is

likely to be larger than this value because the expansion that occurs

in iron during the y -> 6 transformation was not included in the electrical resistivity computations.

The melting point of iron reported in the literature is given in

table 2. All the tabulated values are within 6 K of each other. The

maximum difference of the present value from the values reported by

other investigators is 4 K.

30
o

o

00

o

o

o

o

CD

Ld

ZD

cr

LU

Q_

O LU O H

LO

O

o

OO

rO

0)

(U

0) 43

(1) 4J

}-l o

ou

•H

>^

4-) •HO

nJ&,

COO

on

O

M

P4

l_>i i-lOlAi r'AllOVdVD 1V3H

31
o

o

CO

O

o ^

O ' CO LU — cn

ZD

UJ

CL O

o

ID Ld

IO

o

o

o

rO
TABLE 2

Melting point of pure iron

Investigator Ref

.

Year Melting Point* K

Bristow 11 1939 1806 + 5

Chipman & Marshall 12 1940 1807

Roeser & Wensel 13 1942 1812 + 1

Schofield & Bacon 14 1953 1810 + 10

Present Work 1808 + 5

*A11 temperatures are corrected to the IPTS 1968.

33
Appendix

Table A-1

erimental results on heat capacity and electrical resistivity of iron

Temp. Specimen-1 Specimen2

K c P c P P P

1500 35.87 121.12 36.38 120.13

1520 36.12 121.59 36.62 120.59

1540 36.37 122.05 36.85 121.04

1560 36.62 122.51 37.08 121.49

y-iron 1580 36.88 122.95 37.31 121.92

1600 37.14 123 .39 37 . 54 122.35

1620 37.40 123.82 37.78 122 . 76

1640 37.67 124.25 38.01 123.17

1660 37. 94 124.66 38 . 27 123 . 56

1700 41.37 125.80 41.66 124.87

1720 41.95 126.13 42.22 125.22

1740 42.58 126.45 42.83 125.54

6-iron

1760 43.18 126.71 43.48 125.83

1780 43.81 127.07 44.13 126.08

1800 44.51 127.38 44.86 126.30

Heat capacity in J mol"-^ K electrical resistivity in

10-8 Q, m .

34
5. References

Cezairliyan, A. , Design and operational characteristics of a highspeed (millisecond) system for the measurement of thermophysical

properties at high temperatures, J. Res. Nat. Bur. Stand. (U.S.),

75C (Eng. and Instr.), 7 (1971).

Cezairliyan, A., Morse, M. S., Berman, H. A., and Beckett, C. W.

,

High-speed (subsecond) measurement of heat capacity, electrical

resistivity, and thermal radiation properties of molybdenum in

the range 1900 to 2800 K, J. Res. Nat. Bur. Stand. (U.S.), 74A

(Phys. and Chem.), 65 (1970).

International Practical Temperature Scale of 1968, Metrologia, _5,

35 (1969).

Touloukian, Y. S., and DeWitt, D. P., Thermal radiative propertiesmetallic elements and alloys, Thermophysical Properties of Matter,

Vol. 7, Plenum, New York, 1970.

Pallister, P. R. , The specific heat and resistivity of high-purity

iron up to 1250°C, J. Iron Steel Inst. (London), 161, 87 (1949).

Olette, M. , and Ferrier, A., High temperature enthalpy of gamma and

delta pure iron, NPL Symposium No. 9, Vol. 2, Paper 4 H. (1958).

Bench, W. A. and Kubaschewski, W. , Heat capacity of iron at 800° to

1420°C, J. Iron Steel Inst. (London), 201, 140 (1963).

Kollie, T. G., M. S. Thesis, University of Tennessee (1965).

Kollie, T. G. , Barisoni, M. , McElroy, D. L., and Brooks, C. R.

,

Pulse calorimetry using a digital voltmeter for transient data

acquisition. High Temp. -High Press., 1^, 167 (1969).

35
[10] Kollie, T. G., Private Communication, 1971.

[11] Bristow, C. A., Iron Steel Inst. Spec. Rep., No. 24, 1 (1939).

[12] Chipman, J. and Marshall, S., The equilibrium FeO + H2 = Fe +

at temperatures up to the melting point of iron, J. Am. Chem.

Soc, 62, 299 (1940).

[13] Roeser, Wm. F. and Wensel, H. T., Freezing temperatures of highpurity iron and some steels, J. Res. NBS, 26 , 273 (1941).

[14] Schofield, T. H. and Bacon, A. E., The melting point of titanium,

J. Inst. Metals, 82^, 167 (1953).

I

36
Chapter 3

A SUBSECOND PULSE HEATING TECHNIQUE

FOR THE STUDY OF SOLID-SOLID PHASE TRANSFORMATIONS

AT HIGH TEMPERATURES: APPLICATION TO IROn''

A. Cezairliyan and J. L. McClure

National Bureau of Standards

Washington, D. C. 20234

Abstract

The feasibility of the application of a rapid pulse heating

technique to studies of solid-solid phase transformations at high

temperatures is demonstrated by measurements on iron at the *i ~* ^

transformation point. The measurement system is capable of heating

the specimen from room temperature to its melting point in

less than one second and measuring the pertinent experimental

quantities every 0.4 ms with a full-scale signal resolution of

one part in 8000. Emphasis is placed on the measurement of the

JT-*^ transformation temperature. Two different types of iron yielded

the values 1673 K and 1683 K for the transformation temperature. As

a by-product of the experimental data, an estimate for the transformation

energy is obtained (890 J.mol ^) . The measurements of the normal

spectral emittance (at 650 nm) yielded the values: 0.355 at the

melting point, and 0.361 and 0.368 at the transformation point for

the two types of iron.

This work was supported in part by the U. S. Air Force Office of

Scientific Research.

37
1 . Introduction

Accurate measurements of thermophysical properties at high

temperatures require special techniques to overcome the difficulties

resulting from heat loss, chemical reaction ^evaporation, diffusion,

etc. that are associated with conventional steady-state and quasi

steady-state methods. These problems have largely been solved

by the development of a millisecond resolution pulse heating

technique [1] which is used to measure selected thermophysical

properties of electrical conductors above 1500 K.

The objective of this paper is to demonstrate the applicability

of the pulse heating technique to investigations relating to solidsolid phase transformations. Experimental studies were conducted

in the region of "J-^^ transformation in iron. The results of measurements of transformation temperature are reported and estimation of

transformation energy is discussed. In addition, measurements of

normal spectral emittance (at 650 nm) at the transformation and

melting points are presented.

2. Method

The method is based on rapid resistive self-heating of the

specimen from room temperature to high temperatures (above 1500 K)

by the passage of an electrical current pulse through it. During

an experiment, which usually lasts less than one second, the current

through the specimen, the potential across "t^e specimen, and the

specimen temperature are measured as a function of time and are

38
recorded with a digital data acquisition system. The system

is capable of recording data every 0.4 ms with a full-scale

signal resolution of approximately one part in 8000. Specimen

temperature is measured with a high-speed photoelectric pyrometer

[2]. The details regarding the construction and operation of

the measurement system are given in earlier publications [1, 3],

3 . Measurements

Two types of high purity iron were used; a commercial vacuum

melted electrolytic iron (Ferrovac E)' and Standard Reference

Material 734 electrolytic iron (SRM 7345. A list of the nature

and composition of impurities in each type of iron is given in

table 1.

Experiments were conducted on five specimens in the form of

tubes (T-1 to T-5) and five specimens in the form of rods (R-1 to

R-5) . The tubes were made from rod stock by removing the center

portion using an electroerosion technique. A small rectangular

hole was fabricated in the wall of each tube to approximate blackbody

conditions. A summary of the type and dimensions of the specimens

is given in table 2.

A trade name of Colt Industries Crucible Inc., Syracuse, N. Y. The

identification of this material does not imply recommendation or endorsement by the National Bureau of Standards.

-k-k The SRM 734 electrolytic iron was furnished in rod form by the

Office of Standard Reference Materials (OSRM) of NBS

.

39
Table 1

Impurities in the iron specimens

Impurity Composition, ppm

Ferrovac E SRM 734

Al 100 7

C 60 67

Co 30 70

Cr 100 72

Cta 10 58

Mn 10 57

Mo 10 50

Ni 300 410

30 25

Pb 6 < I

S 70 59

Si ' 80 80

Sn 10 < 5

Ti 6

V 40 6

W 100 < 1

«2 1 < 5

^2 4 < 20

^2 260 < 70

Total 122 1 < 1069

40
Table 2

Suimnary of the type and dimensions of the iron specimen

S pecimen

Geometry Designation Type of Material Nominal Dimension

Tube Ferrovac E Length: 89 minn

r)iit"<?idp DiamptPT* fi U. mm

Wall Thickness: 0.5 mm

Sighting Hole: 0.5 x 1 mm

T-3,T-4,T-5 SRM 734

Rod R-j,R-2,R-3 Ferrovac E Length: 89 Tn-m

R-4, R-5 SRM 734 Diameter: 3.2 mm

41
33NVia\/d

42
An oscilloscope trace photograph, showing the variation of

the specimen radiance as a function of time during a typical

heating experiment, is presented in figure 1. The transformation

is indicated by the plateau on the left. The higher plateau

on the right indicates melting of the specimen.

Preliminary work had indicated that the abrupt volume changes

that occur at tfe and the^-*^^ transformations distort the

specimen to such an extent that multiple heatings through the

transformation region become undesirable. For this reason^in most

experiments the specimen was heated from room temperature through

the transformation point to its melting point.

Two successive heatings were attempted on two tube-shape

specimens, T-1 (Ferrovac E) and T-5 (SRM 734) . The attempt on

T-1 was not successful. Two successful heatings were conducted

on T-5; a low range experiment to 1710 K and a high range experiment

to melting. Figure 2 shows the two heating curves for this specimen.

The temperature plateau at approximately 1683 K is due to the

solid-solid transformation. The higher temperature plateau on

the right corresponds to melting of the specimen. A typical

heating curve for a rod-shape specimen is shown in figure 3.

The duration of the current pulses in the experiments on tubeshape specimens ranged from 500 to 900 ms while that in the

experiments on rod-shape specimens ranged from 800 to 1900 ms

.

All the experiments were conducted with the specimen in a vacuum

environment of approximately 10 ^ torr.

43
o o o o o o o o

00 CD LO

>i '3dniVd3dlAI31

44
CO

LO

LO

CO CO

LO

LO LO

45
4 . Results

All the results reported in this paper are based on the

International Practical Temperature Scale of 1968 [4].

The measured temperatures in the transformation region are

shown in figures 4 and 5 for the tube-shape specimens and in

figures 6 and 7 for the rod-shape specimens. Most of the results

show an overshoot of approximately 2-5 K at the beginning of

the plateau. For each experiment, an average transformation

temperature was obtained by averaging the temperatures at the

plateau. The results are presented in tables 3 and 4 for tubeand rod-shape specimens, respectively. In the same tables heating

rate of the specimens and the maximum deviation of an individual

temperature from the average are also given.

Average transformation temperature obtained from measurements

on tube-shape specimens are: 1673 K for Ferrovac E iron and

it- 1683 K for SRM 734 iron. Average radiance temperature at the

transformation point obtained from measurements on rod-shape specimens

are: 1555 K for Ferrovac E iron and 1562 K for SRM 734 iron. The

results of individual experiments are shown, in graphical form,

in figure 8. .

''radiance temperature (sometimes referred to as brightness temperature) is the apparent temperature of the specimen surface as measured by the pyrometer. In order to obtain specimen true

temperature normal spectral emittance of the specimen surface at the

pyrometer wavelength is required.

46
lon<

cr

GO

cr<

LU

o

>o

m

C

0)6

•HO

0)CL

CO

0)p.

x:

I

3

CO0)>

o

bOC

•1-1

0) re

M

P4

CO6

CO

CO

00

4-J •-Id

3

0)e

6

o

COC

^1

M

C

•r-l

!-l

XICo

S-i

>1 '3dniVd3dlAI31

47
1700

1680 -

1660-

170 0-

1680-

166 0-

1700-

1680-

1660-

1700

1680-

1660-

FIGURE 5

TIME, ARBITRARY UNIT

Heating curves for the tube-shape specimens of SRM 734

iron durine the'Jf-*^ transformation. Cone time unit = 0.833 ms)

48
o

>

o

(-1 u

d)

o

cnC

0)e

ec

05 >

0) <

DC

CO

1 \- X) CD ro <

LlT

M-i

>i '3dnivd3diM3i 30Nviava

49
o o o o o o

0^ lO (D h- lO

lo lo in ID lo

>i '3dniVa3dlAI31 30NViaVd

50
Table 3

Average^trans formation temperature and heating

rate just before transformation for the tube-shape iron specimens

Specimen Type Avg. Trans f. Temp., KMax, Dev. from

Average, K

Heating Ratel

K . s-1

T-1 Ferrovac E 1673.4 0.5 3200

T-2 Ferrovac E 1672.1 1.2 3350

T73 SRM-734 1681.5 1.2 3000

T-4 SRM-734 1681.6 1.1 2220

T-5 (low) SRM-734 1683.1 0.5 2630

T-5 (high) SRM-734 1683.8 0.5 2810

Table 4

Average radiance temperature at the 'X'-*^ transf ormation

point, heating rate just before transformatioUj and average radiance temperature

at the melting point for the rod-shape iron specimens

Specimen Type Avg . Rad . Temp

.

at Transf., K

Max. Dev. from

Average, K

Heating Rate

K.s-1

Avg. Rad .Temp

.

at Melt., K

R-1 Ferrovac E 1555.0 0.5 2800 1667.2

R-2 Ferrovac E 1557.1 1.1 3190 1666.9

R-3 Ferrovac E 1553.6 1.0 1270 1667.3

R-4 SRM 734 1561.0 1.4 1200 1667.4

R-5 SRM 734 1562.3 1.6 1180 1666.6

51
UJ

cchi

CL

U

680 -

670 -

1670

UJ

z>

crUJ

Q.

iij

IxJ U

<

660

560

1550

MELTING

_ TRANSFORMATION

ROD

SPECIMEN NUMBER

FIGURE 8 Measured radiance temperatures at the transformation

and the melting points and measured temperatures at the

transformation point of iron.(#} FerrovdC E ,(o) SR/\/| 734

52
The radiance temperature at the melting point for the rod-shape

specimens was also measured. The results on five specimens (table

4) yield an average value of 1667 K for the radiance temperature

at the melting point of iron. The maximum difference of an

individual result from the average is 0.4 K.

An attempt was made to determine the energy of the

transformation from the data taken on the rod-shape specimen (R-4)

.

From the temperature profile around the transformation point, the

duration of transformation was obtained. Integration of the power

absorbed by the specimen over the transformation period yielded

the transformation energy . 890 J.mol .

The radiance temperature corresponding to the transformation

and melting points for the rod-shape specimens in conjunction with

similar data on the tube-shape specimens were used to determine

the normal spectral emittance at the transformation and the melting

points. The melting point of iron was taken as 1808 K [5]. The

results of normal spectral emittance are given in table 5 and

correspond to 650 nm (the effective wavelength of the high-speed

pyrometer with a bandwith of 10 nm) . The average values of the

normal spectral emittance are: 0.355 at the melting point, and

0.361 and 0.368 at the transformation points for Ferrovac E and SRM

734 irons, respectively.

53
Table 5

Normal spectral emlttance (at 650 nm) of iron at the'3-»'^ trans formation

point and at the melting point

Specimen Type Normal spectral emittance

at transformation point

Normal spectral emittance

at melting point

R-1 Ferrovac E 0.367 0 . J56

R-2 Ferrovac E 0.374 0 .355

R-3 Ferrovac E 0.362 0.356

R-4 SRM-734 - 0.359 0.356

R-5 SRM-734 0.363 0.354

Table 6

Temperature and energy of the '3^^^trans formation

in iron as reported in the literature

Investigator Ref

.

Year Transf. Temp.

K

Transf. Energy

J . mol"^

Bristow 6 1939 1663

Boulanger 7 1956 1667

Pattison and Willows 8 1956 1675 870

Olette and Ferrier 9 1958 1675 1170

Ferrier and Olette 10 1962 1100

Dench and Kubaschewski 11 1963 1666 837

Braun and Kohlhass 12 1965 850

/)H t<mp«rd.'tufcs art aorrected -to XPTS

54
A detail analysis of errors in such experimental quantities

as temperature, voltage and current measured u.oing the present

pulse heating system has been given in an earlier publication

[3]. The errors in the final results depend on the errors in

the experimental quantities as well as on the errors in the

method of determining the results from the experimental data.

The estimates of sources and magnitudes of the above errors yield

the following errors in the reported results: 10 K in transformation

temperature, 5 K in radiance temperature, 10% in transformation

energy, and 3 and 5% in normal spectral emittance at the melting

point and transformation point respectively.

5 . Discussion

Experiments on both tubes and rods indicate that the

transformation during heating occurs at a lower temperature in

Ferrovac E iron "tfian Jtv SRM 734 iron (figure 8). The reason

for this effect is not" known, but it may be due to the relative

composition and nature of the impurities in each type of iron.

TheT*^ transformation temperature reported in the literature is

given in table 6. The maximum disagreement in these values is

12 K. The value obtained for Ferrovac E iron is in good agreement

(within 2 K) with the values reported by PattisOn and Willows C^J >

and Olette and Ferrier [9]. The value obtained for SRM 734 is 6

to 18 K higher than values reported by other investigators.

55
The results reported in table 4 for the Ferrovac E rods seem

to indicate that the transformation temperature increases with the

specimen heating rate. This point was not studied in detail^

but it is possible that the heating rate would have some effect

on the transformation temperature. A comparison of the experiments

on tubes (T-3 and T-4) do not indicate any heating rate effect

.

The results of figure 5 show that the degree of overshoot at the

beginning of the transformation may be dependent on the heating

rate. As it may be seen from table 6, considerable disagreements

(up to 30%) exist in the values of transformation energy reported

in the literature. The closest agreement (within 3%) of the present

value is with that reported by Pattison and Willows [8].

The literature contains only a few results on normal spectral

eraittance of iron at the temperatures reported here. Bidwell [13]

has reported a value of 0.38 at the melting point of iron and Burgess

and VJaltenberg [14] have reported a value of 0.360 at about 1800 K.

Near the transformation point the Burgess and Waltenberg have

reported a vafufi ol^ . The results of Wahlin & Knop [15]

show an increase of approximately 4. 5% in the normal spectral emittance

during the transformation. At 1675 K this increase would correspond

to an increase in radiance temperature of approximately 6 K. Such

as increase is not evident in figures 6 and 7.

56
The experimental results obtained in this work demonstrate

the feasibility of using the pulse heating technique to study

solid-solid phase transformations in electrical conductors at high

temperatures. The results indicate that with proper selection of

heating rates and refinements in time resolution^ transformation

temperatures and energies may be accurately determined and in addition

useful information regarding transformation kinetics may be obtained,

Ac know1 edg eme nt

The authors express their gratitude to Dr. C. W. Beckett for

his encouragement of research in high-speed thermophysical measurements and to Mr. M. S. Morse for his help with the electronic

ins trumenta ti on

.

57
6 . References

A. Cezairliyan, Design and operational characteristics of

a high-speed (millisecond) system for the measurement of

thermophysical properties at high temperatures, J. Res.

Nat. Bur. Stand. (U. S.), 1971, vol. 15C, (Eng. and Instr.),

pp. 7-18.

G. M. Foley, High-speed optical pyrometer, Rev. Sci. Instr.,

1970, vol. 41, pp. 827-834.

A. Cezairliyan, M. S. Morse, H. A. Berman, and C. W.,

Beckett, High-speed (subsecond) measurement of heat capacity,

electrical resistivity, and thermal radiation properties

of molybdenum in the range 1900 to 2800 K, J. Res. Nat. Bur.

Stand. (U. S.), 1970, vol. 74A, (Phys. and Chem.), pp. 65-92..

International Practical Temperature Scale of 1968, Metrologia,

1969, vol. 5, pp. 35-44.

and

A. Cezair liyan,^J . L. McClure, Thermophysical measurements

on iron above 1500 K using a transient (subsecond) technique,

J. Res. Nat. Bur. Stand. (U. S.), Section A, in press.

C. A. Bristow, Iron Steel Inst. Spec. Rep., No. 24, 1939,

pp. 1-8.

C. Boulanger, Contribution a 1 'etude de la purification

du fer et a la determination de ses points de transformation.

Rev. Metallurg., 1956, vol. 53, pp. 311-319.

58
[8] J. R. Pattison^ and P. W. Willows, The enthalpy of pure

iron, J, Iron Steel Inst. (London), 1956, vol. 183, pp. 390-403.

[9] M. Olette, and A. Ferrier, High temperature enthalpy of

gamma and delta pure iron, NPL Symposium, 1958, No. 9, Vol. 2,

Paper 4H

.

[10] A, Ferrier, and M. Olette, Measure de la capacite calorifique

du. fer entre 910 et 1937°C, Compt. Rend., 1962, vol. 254,

pp. 2322-2324.

[11] W. A. Dench, and W. Kubaschewski, Heat capacity of iron at

800°C to 1420°C, J. Iron Steel Inst. (London), 1963, vol. 201,

pp. 140-143.

[12] M. Braun, and R. Kohlhass, Die spezifische Warme von

Eisen, Kobalt und Nickel im Bereich hoher Temperaturen, Phys . .

Stat. Sol., 1965, vol. 12, pp. 429-444.

[13] C. C. Bidwell, Radiation from solid and molten iron, Phys.

Rev., 1913, vol. 1, pp. 482-^53.

[14] G. K. Burgess, and R. G. Waltenberg, The emissivity of

metals and oxides, Nat. Bur. Std. Bull., 1915, vol. 11,

pp. 591- 6*05.

[15] H. B. Wahlin, and H. W. Knop Jr., The spectral emissivity

of iron and cobalt, Phys. Rev. 1948, vol. 74, pp. 687- 659.

59
chapter 4

MEASUREMENT OF MELTING POINT, NORMAL SPECTRAL

EMITTANCE (AT MELTING POINT) AND ELECTRICAL

RESISTIVITY (NEAR MELTING POINT) OF SOME

REFRACTORY ALLOYS"

•, Ared Cezairliyan

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

Abstract

A subsecond duration pulse heating method is used to measure

the melting point, normal spectral emittance (at the melting point,

corresponding to 650 nm) , and electrical resistivity (near the

melting point) of the following refractory alloys: 90 Ta-lOW,

99 Nb-lZr, and 80 Nb-lOTa-lOW (numbers indicate nominal composition

in percentage by weight) . The melting point and the normal spectral

emittance are: 3286 ± 15 K and 0.396 for 90Ta-10W, 2737 ± 10 K and

0.351 for 99Nb-lZr, and 2814 ± 10 K and 0.333 for 80Nb-10Ta-10W . The

inaccuracy of the normal spectral emittance and electrical resistivity

results is estimated to be 3% and 0.5%, respectively.

This work was supported in part by the U. S. Air Force Office of

Scientific Research.

60
1. Introduction

A millisecond resolution pulse heating technique was used

earlier for the measurement of the melting point, normal spectral

emittance (at the melting point), and electrical resistivity (near

the melting point) of niobium [1]''". In the present study, the

same technique is used for similar measurements on the refractory

alloys: 90Ta-10W, 99Nb-lZr, and 80Nb-10Ta-10W (numbers indicate

nominal composition in percentage by weight)

.

The technique is based on resistive self-heating of the specimen

from room temperature to its melting point in less than one second

and measuring current through the specimen, potential difference

across the specimen, and specimen temperature. These quantities

are measured and recorded digitally every 0.4 ms with a full-scale

signal resolution of approximately 1 part in 8000. The details

regarding the construction and operational characteristics of the

measurement system is given in an earlier publication [2].

2. Measurements

Melting point and electrical resistivity measurements were

performed on specimens in the form of tubes having the following

nominal dimensions: length, 102 mm; outside diameter, 6.3 mm; and

thickness, 0.5 mm. A small rectangular hole (0.5 x 1 mm) fabricated

Figures in brackets indicate the literature references at the end

of this paper.

61
in the wall at the middle of the specimen approximated blackbody

conditions for temperature measurements. Normal spectral emittance

measurements were performed on specimens in the form of rods having

the following nominal dimensions: length 76 mm; diameter, 3.1 mm.

The surfaces of all specimens were polished. The composition of

the alloys is given in table 1.

Before starting the experiments, the specimens were heat treated

by subjecting them to 30 heating pulses in the temperature range

2200 to 3000 K. The experiments were conducted in a vacuum

-4 -5

environment of 10 to 10 torr. Duration of current pulses

(duration for the specimen to heat from room temperature to its

melting point) was in the range 380 to 750 ms

.

For each alloy two experiments were performed for the

determination of the melting point and one experiment was performed

for the determination of nprmal spectral emittance and electrical

resistivity. Designation of the specimens and the results of

some of the measurements are given in table 2. All measurements

reported in this paper are based on the International Practical

Temperature Scale of 1968 [3].

Table 1 Composition of the alloys

Alloy Major Constituents Total Impurities

(7o by weight) (7o by weight)

90Ta-10W Ta,90.45; W, 9.45 0.10

99Nb-lZr Nb, 98.78; Zr, 1.05 0.17

80Nb-10Ta-10W Nb, 80.94; Ta, 9.70; ^, 9.30 0.06

62
• • 00 CN > • • • TO O o o

4-1

Q

cn c e •r-i O CN CN H D. • * ^ • • T3 00 CO ON

H) t)0 00 CN o c £^ E 00 <} <f j c •1-1 Csl CN CN 1 TO 4-1 •H 1—1

T3 OJ ' TO S S-i Oi 0) B

4-1 ^ D

TO

H vO 1—1

13 CN CN 00

14-1 Q o • c > CO CO CO

TO

4-1

G 1—1 o 1—1 —

o o

CO

4J CNl c C o 1—1 o '

' CO

•1-1 • " i 0 4J CN

P-I .—J

C 00 00 r~i CO 1—1 1—4

0) •H CM CM 100 00

ts o CO CN CN CN CN c P-I T-l

4-1 1—1 I OJ e f E CN CO <t 1—1 o

CN CM CN CO

O

(U4J o O

OO

OOO

O o w 00 o 00 o> CO CO \D I—1

3<u <f r-^ CO CO CO VD

>.u

4-1 o aj OJ 0) 0) 0) "O T5 e 3 o 3 3 o 3 3 0 o 4-1 4—1 4-) 4-) 4-i U

<u o S o c o 1—1 1—1 1—1 0) IS S-I S-I 1 1 1 o CN TO TO TO c o o o H H E-i to 1—1 t—' ^

4J 1 1 1 1 1 o o O

(0 TO TO TO

X I—1 1—i

XI

H

H S 1 1 CO O O O o> ON 1 ON o> o> o> ON o o o

CO 00 00 c

0)e S-I 0) O

(U E 1—1 CM CO CO

D. 3

cn 3

63
2.1. Melting Point

Temperature of the tubular specimens was measured near and

during the initial melting period. A plateau in temperature

indicated the transition from solid to liquid phase. Measured

temperatures at the plateau for specimens are shown in figure 1.

The melting point for each specimen was obtained by averaging

the temperature points on the plateau. The results are presented

in table 2. The standard deviation of an individual point was

in the range 0.3 to 1.9 K. As may be seen in table 7., the

difference between the results for the two specimens of the same

alloy was in the range 0.6 to 3.3 K. The average melting points

of the alloys are given in table 3

.

2.2. Normal Spectral Emittance

Normal spectral emittance at the melting point of each alloy

was determined from the measurements of the radiance temperature oJ

the specimen (in rod form) and the average melting point obtained

from the measurements on the tubular specimens. Based on the

Wien radiation equation, the relation between emittance and

temperature is

e =

r ^2 ,1 1 V . expL TT- (- - - ) J At T

S m

l5 where p the normal spectral emittance, T the melting point, T m

the radiance temperature, the second radiation constant

-2

(1.4388 X 10 m K), and > the effective wavelength of the optical

64
Table 3

Melting point (T^) > radiance temperature (T^) at the melting

point, and normal spectral emittance (e) at the melting point of

refractory alloys

Substance T (K)

m

T^(K)

(at 650 nm)

e (at 650 nm)

90 Ta - 10 W 3286 2889 0.396

99 Nb - 1 Zr 2737 2423 0.351

80 Nb - 10 Ta - 10 W 2814 2469 0.333

65
I I 1 1 1 1 o o yu iQ - lu

w

3288 0

° SPECIMEN-I

• SPECIMEN-2

—

o

o . O o 0 o o • 3286 1 • •

•

• o ' • . • 3284

• • 2K

•

1 1 1 1 1 1

281 6

281 4

281 2

2740

2738

2736

2734

1 1 1 1 1 1 o

o ° o 80Nb-IOTa--low

- o ° ° o „ ° o° ° °

° °SPECIMEN-7

•SPECIMEN-8 - 2814.4

•

•

•

•

• • •

•

•

• •

• •

• •

• 2K

1 1 1 1 1

1

99 Nb- I Zr

° SPECIMEN -4

• SPECIMEN -5

o_._ 2737.3

2K

0 10 20 30

TIME. ARBITRARY UNIT

FIGURE 1. Variation of the temperature of various refractory

alloys as a function of time at their respective

melting points (1 time unit = 0.833 ms) .

66
system. The measurements were performed at 650 nm which corresponds

to the effective wavelength of the pyrometer's interference filter.

The bandwith of the filter was 10 nm. The circular area viewed

by the pyrometer was 0.2 mm in diameter.

The experimental results of the radiance temperature of 90Ta-10W

and 99Nb-lZr alloys near and at the respective melting points are

shown in figure 2. Similar results for the alloy 80Nb-10Ta-10W are

shown in figure 3. It may be seen (figure 2) that both 90Ta-10W

and 99Nb-lZr samples behaved normally, that is, radiance temperature

increased and exhibited a plateau upon melting. However, 80Nb-10Ta-10W

results (figure 3) do not indicate the existence any well-defined

plateau, with the exception of a short segment immediately after

reaching the melting point. The results of radiance temperature at

the melting point of the alloys and corresponding values for the

normal spectral emittance are given in table 3.

2.3. Electrical Resistivity

Electrical resistivity of the tubular specimens was calculated

using the relation = RA/L, where R is the resistance, A the crosssection area, and L the length of the specimen between the potential

probes. The dimensions were based on their room temperature values;

the cross -section area was determined from the measurement of weight

A

and density. The results for the electrical resistivity of the alloys

near and at their respective melting points are shown in figures 4,

67
UJ

(T

liJ CL

LU

hLiJ O

<

<

or

2890

2880

2870

T

^2888.7

5K

90 Ta - 10 W

SPECIMEN -3

1 1

• •

1

- • • • • • . •

• •_ •

2420

• \2423.2

•

•

• 5K

2410 • 99Nb - 1 Zr

SPECIMEN-6

•

1 I 0 10 20 30

TIME, ARBITRARY UNIT

FIGURE 2. Variation of the radiance temperature (at 650 nm) of

the alloys 90 Ta - 10 W and 99 Nb-1 Zr as a function

of time near and at their respective melting points

(1 time unit = 0.833 ms)

68

40
in

o en

1

1 -z:

1 LU

o

o

LU

CL

o

CO

00

00

oS

CD

(\J

/

•I

o

CO

o

CO

o

CO

o

LO

CO

69
5 and 6. The values for the 100 K range below the melting points

are given in table 4. The results indicate that electrical

resistivity of the alloys 90Ta-10W (figure 4) and 99Nb-lZr (figure

5) changed almost abruptly at the start of melting. However,

in the case of the alloy 80Nb-10Ta-10W (figure 6) the change in

the electrical resistivity was gradual, starting approximately 15 K

below the melting point. Below the melting point electrical

resistivity for each alloy was fitted, using the least squares

method, to a linear function in temperature. In all cases, the

standard deviation of an individual point from the smooth function

was less than 0.06%.

2.4. Estimate of Errors

Sources and estimates of errors in experiments similar to the

ones conducted in this study are given in earlier publications [4,

51. Specific items in the error analysis were recomputed whenever

the present conditions differed from those in the earlier publications.

2 As Lt may be seen from table 2^ the imprecision of true

(blackbody) and radiance temperature measurements was 0.3-1.9 K and

0.2-0,8 K, respectively. A somewhat higher imprecision in the

case of the blackbody temperature measurements may be attributed

to the movements of the sighting hole in the specimen during melting.

Imprecision refers to the standard deviation of an individual point

as computed from the difference between measured value and that from

the smooth function obtained by the least squares method.

70
Table 4

Electrical resistivity of some refractory

alloys near their melting points

Alloy Temp

.

Elec. Res.

(K) (10"^ Q m)

90 Ta - 10 W ' 3180 110.88

3230 112.03

3280 113 .17

[

i 99 Nb - 1 Zr 2630 87.06

1

i 2680 88.25

i

1 2730 89.43

80 Nb - 10 Ta - 10 W 2700 88.49

2750 89.70

2800 90.92

Based on ambient temperature (298 K) dimensions.

71
o

o

ro

\

1 o

1 O

o

cn

UJ

a.

o

ID

C\J

rO LU

cr

or

LU

Q_

LU

O

O

C\J

ro

H

O

O

u

o

>^

4J

•r-l >

J-) CO

•I-l w

0)

o

•l-l

i-l o

<u

1-4

CVJ H

00 CD CM O

uJUg.oi'AiiAiisisad nvoidio3n3

72
73
T T

o 1

1 LU

o o

'Nb- SPE

o

00

\

\

\

\

\

\

\

CO \

\

1

CD

(J) CD

C\J O

cn

^Uq-OI *AllAllSIS3d nV3ldl03n3

00

CO

74
The inaccuracy of the melting points is estimated to be approximately

10 K for the alloys 99Nb-lZr and 80Nb-10Ta-10W, and 15 K for

90Ta-10TAj.

The combination of inaccuracies in the temperature measurements,

taking into account likely common errors in both blackbody and

radiance temperature measurements, indicates that the inaccuracy in

the normal spectral emittance is approximately 37o.

The imprecision of the electrical resistivity results is 0 . 05%

and the estimated inaccuracy is 0.5%.

3. Discussion

The agreement of the melting points of the specimens of the

same alloy were in the range 0.6 to 3.3 K, indicating reasonably

good reproducibility of the measurements. Radiance temperature

and electrical resistivity of the binary alloys 90Ta-10W and

99Nb-lZr behaved normally, that is exhibited an almost abrupt change

at the beginning of melting. However, in the case of the ternary

alloy 80Nb-10Ta-10W the change in electrical resistivity was more

gradual, starting approximately 15 K below the melting point.

Radiance temperature of the ternary alloy exhibited a peak before

melting, a short plateau immediately after the start of melting,

and a continuous increase as melting progressed. The peak may be

due to the roughness of the solid surface, and the increase in

radiance temperature during melting may be attributed to the increase

3

Inaccuracy refers to the estimated total error (random and systematic)

.

75
of the normal spectral emittance during melting. With the present

system it was not possible to follow the entire melting process

because the specimens collapsed and opened the electrical circuit

prior to the completion of melting.

76
4. References

Cezairliyan, A., Measurement of Melting Print, Normal Spectral

Emittance (at Melting Point), and Electrical Resistivity

(above 2650 K) of Niobium by a Pulse Heating Method, High

Temperatures - High Pressures, 4^, 453 (1972).

Cezairliyan, A., Design and Operational Characteristics of a

High-Speed (Millisecond) System for the Measurement of Thermophysical Properties at High Temperatures, J. Res. Nat. Bur.

Stand. (U. S.), 75C (Eng. and Instr.), 7 (1971).

International Practical Temperature Scale of 1968, Metrologia,

5, 35 (1969).

Cezairliyan, A., Morse, M. S., Berman, H. A., and Beckett,

C. W., High-Speed (Subsecond) Measurement of Heat Capacity,

Electrical Resistivity, and Thermal Radiation Properties of

Molybdenum in the Range 1900 to 2800 K, J. Res. Nat. Bur.

Stand. (U. S.), 74A (Phys. and Chem.), 65 (1970).

Cezairliyan, A., Morse, M. S., and Beckett, C. W., Measurement of Melting Point and Electrical Resistivity (Above

2840 K) of Molybdenum by a Pulse Heating Method, Rev. Int.

Hautes Temper, et Refract., 7, 382 (1970).

77
Chapter 5

THE ENTHALPY OF FORMATION OF MoF^(c) BY SOLUTION CALORIMETRY

:

PRELIMINARY ANALYSIS OF EXPERIMENTS

R. L. Nuttall, M. V. Kilday^ and K. L. Churney

1. Introduct ion

This laboratory is engaged in a program to determine the enthalpy

of formation of the lower fluorides of molybdenum. The compound of

current interest is MoF^(c). The method finally selected for determining the enthalpy of formation of MoF^(c) was to measure the heat of

solution of MoF (c) in a solution of HF and XeO„ and of MoF, (-^) in

-> J 6 a similar solution to give the same concentrations of all the aqueous

species in the two final solutions. This method requires an accurate

knowledge of the enthalpy of formation of M.oF^(l) and XeO^ (solution).

The enthalpy of formation of MoF^(g) was measured by Settle et.

al. [1] by direct combination of the elements in a combustion bomb.

A recalculation of their results [2] combined with the selected value

of the heat of vaporization [3] of KoF^(£) gives a value of -1585.53 ±

0.92 kJ mol"l (-378.95 + 0.22 kcal mol"''") for the enthalpy of formation

of MoF. (i) .

Previous measurements in our laboratory [4] of the heats of

solution of MoF^(Ji) and Mo02(c) in basic solution gave a value of

z^°[MoF,(i)] of -1574.98 ± 2.18 kJ mol"-*- (-376.43 ± 0.52 kcal mol"-^).

r o The assigned uncertainty is due primarily to the total estimated

uncertainty of our experimental measurements^ ±2.09 kJ mol ^ , and

does not include any uncertainty in the value taken for the enthalpy

78
of formation of F"(aq) of -332.63 kJ mol"''" (-79. "jO kcal mol"''') [5].

Using a value of AH°[F"(aq)] of -335.35 ± 0.65 kJ mol~^ (-80.15 ±

0.16 kcal mol ''') selected in a more recent critical evaluation [6],

we obtain a value for AH°[MoF, (^)] from our previous experimental

measurements of -1591.30 ± 3.72 kJ mol"""" (-380.33 ± 0.89 kcal mol"-"").

The agreement between the latter and the value derived from the

measurements of Settle et. al. is considered to be satisfactory.

XeO^(aq) was originally chosen as an oxidizing agent to determine if a reaction scheme involving basic rather than acidic solution

could be characterized. The negative results of this preliminary

investigation are discussed briefly in the Appendix. In dilute acid

solutions, Xe02(aq) is stable and forms the single product Xe(g) in

an oxidation-reduction reaction [7,8]« The enthalpy of formation of

[Xe02«96.15 (i)] in a dilute acid solution was determined by

O'Hare, Johnson, and Appelman [7]. They report a value of 418.15 ±

1.00 kJ mol""*" (99.94 ± 0.24 kcal mol"''") and a heat of dilution to

[XeO^- 14,000 n^O(Jl)] of 0 ± 126 J mol"''" (0 ± 30 cal mol"''").

2 . Reaction Schemes of Solution Experiments

A scheme of reactions that yields the enthalpy of formation of

MoF^(c) is as follows:

MoF^(c) +[(n^ + l/6)XeO^ + (n^ - 5)HF + (n^ + 7/2)H20] (soln I) = (1)

[H^MoO^ + n^XeO^ + n^HF + n2H20] (soln II) + 1/6 Xe(g) - AH^^

79
MoF^(^) + [n^XeO^) + (n^ - 6)HF + (n^ + 4)R20] (soln III) = [II] - AH^

[I] + ooH^OCje) =

(n^^ + l/6)Xe02(aq) + (n^ - 5)HF(aq) + (n^ + 7/2)H20(aq) - Zffl^

[III] + coH^OC^) = n^XeO^Caq) + (n^ - 6)HF(aq) + (n^ + 4)H20(aq) -

Adding equations (1) - (2) - (3) + (4) gives

MoF^(c) + l/eXeO^Caq) + HF(aq) = MoF^ (^) + 1/2H20(^) + l/6Xe(g) - AH^

where AH^ is given by

AH = AH, - Z^ffl_ - AH„ + AH.

5 12 3 4

The heat of formation of MoF^ (c) is calculated from the expression

AH°[MoF^(c)] = AH°lMoF^(^)] + 1/2AH° [H20(je) ] - 1/6AH° [XeO^ (aq) ] -

AH°[HF(aq)] - Zffl^

In this scheme Zffl^ and ZvH^ are measured experimentally. AH^ and

are estimated from literature values [ 9 ] of the heat of dilution data

for HF solutions and eq. (8).

n + 7/2 n + 4 n + 7/2

In eq. (8)^ (^) the apparent molal enthalpy of HF in a solution

with X moles of water. We have assumed there is no interaction between

XeO^ and HF and that the heat of dilution of XeO^ is zero. Experiments

will be carried out to test the validity of these assumptions.

80
3 . Experimenta

1

a . Materia Is

MoF- was purchased from Ozark-Jfehoning Company. It was purified

by trap-to-trap distillation and was then transferred by distillation into

Kel-F (polychlorotr if luoroethy lene ) bulbs for introduction into the

calorimeter. The sample bulbs have been described previously [4].

Samples of MoF^ which had received similar treatment were analyzed

by freezing point lowering by Krause [10] who found an impurity of

0.03 mol percent. The nature of the impurity was not determined.

MoF^ was made by reacting MoF^ with molybdenum powder at 150 C.

The MoF^ was then distilled off and purified by trap-to-trap distillation

at 90 C with an atmosphere containing MoF at a partial pressure of

D about 17 Pa. The light yellow crystalline material was transferred into the platinum calorimeter sample holder in an atmosphere

of dry Argon.

b . Calorimeter

The calorimeter used was a precise adiabatic calorimeter with

an all-platinum interior. Its description and operation are given

elsewhere [ 11] .

4. Results

a . Experimental Results

Results of the calorimetric measurements are given in Table 1

for the solution of MoF^ and in Table 2 for the solution of MoF,.

D b

In these tables n^^ n^, and n^ are the coefficients in equations (1)

* Disclaimer

81
and (2). e is the energy equivalent of the calorimeter obtained as the

mean of fwo calibration experiments^ one before and one after the

reaction experiment. AT^ is the corrected temperature rise of the

calorimeter. Q is the measured heat of reaction equal to AT^ times e. T is the average temperature of reaction.

The data analysis given here is preliminary and will be refined

at a later time. Based on the data in Table 1, we estimate a heat

of reaction (1) of

-AH^ = 340.01 ± 1.0 kJ mol"-^ (81.26 ± 0.5 kca 1 mol""^)

The mass of sample given in Table 2 was determined by weighing

the plastic bulb (with its sidearm attached), filling with MoF^(i),

sealing by pinching off the sidearm and weighing the filled bulb

and then the separated sidearm. An alternate method of determining

the sample size is to weigh the filled bulb and then weigh the empty

bulb removed from the calorimeter after the reaction. These two

procedures give sample weights that differ by milligrams.

Using sample weights determined by the second procedure we

estimate values of

-AH2 261.43 kJ mol"-^ (62.48 kcal mol"-*")

Std. dev. 2.65 kJ mol"''" (0.63 kcal mol"-*-)

Std. dev. mean = 0.88 kJ mol'''' (0.21 kcal mol"''')

The corrections for the vaporization of water by the Xe (g) evolved and

the Ar(g) liberated when the sample holder is opened are approximately

0.017o and 0.01-0.03% of AH^^, respectively. They are small in comparison

to other systematic errors in the final value obtained for ^£[MoF^(c)]

and have not been included in these results.

82
to be compared with the values given in Table 2. We thus have a systematic

uncertainty in of about one kilocalorie per mole. We are now

analyzing the final solutions from the calorimetric experiments to

obtain a value for the amount of sample used. We expect to be

able to reduce the uncertainty in the value of •

We estimate an average value of ~ 259.39 + 4. kJ mol ^

(62.00 + 1. kcal mol"''").

b. Evaluation of (m^ - M^)

Using average values of n^ and n^ from Tables 1 and 2 and

values of cp^ from Parker [10] we get n^ = 20.43, n^ = 14930.

(n^ + 7/2) /(n^ - 5) - 968

(n^ + 4)/(n2 - 6) = 1035

cpj^(968) = 2975 cal mol""^

cp^ (1035) - 2955 cal mol"-*"

Li

From equation (8)

ZNH, - = 13.65 ± 0.84 kJ mol"-*- (3.26 + 0.20 kcal mol""^)

4 3

c. Evaluation of Zffl^

From equation (6) and using the above values of AH^, ^2-' ^^'^

(AH^ - AH^) we obtain for

- -340.01 + 259.35 + 13.65

= -67.01 ± 4.6 kJ mol""*" (-16.02 ± 1.1 kcal mol"-*")

83
d. Evaluation of the enthalpy of formation of MoF^, Zffl°[MoF^]

Using equation (7) and the following selected values for

enthalpies of formation the enthalpy of formation of MoF^ can be

calculated.

AH°[H20(^)] = -68. 135 ± .05 kcal mol""^ [5]

Zffl|[XeO^(aq)] = 99.94 ± .24 kcal mol""*" [8]

AH°[HF(aq)] = -80.15 ± .12 kcal mol""^ [6]

z^H°[MoF^(Ji) = -378.95 ± .22 kcal mol"-*" [1],[2]

= -16.02 ± 1.1 kcal mol""*"

From these values a value is obtained for the difference in enthalpy

of formation of MoF^ and MoF, of

5 6

^°[MoF^(c)] - AH°[MoFg(^)] = 189.79 ± 4.6 kJ mol"^

(45.36 ±1.1 kcal mol'"^)

and for the enthalpy of formation of MoF^ of

AH°[MoF^(c)] = -1395.74 ± 4.6 kJ mol"-*-

(-333.59 + 1.1 kcal mol"-*-)

In our experiments we have not controlled the stoichiome try to

comply exactly with the reaction scheme shown in equations (1) through

(5). The effects of differing concentrations of final solutions on

the heats of reaction appears to be small compared to other uncertainties

of the results and have been neglected in this analysis. In our final

data analysis attempts will be made to evaluate the effects of differing

concentrations and temperatures between the individual experiments.

84
5 . References

[1] Settle, J. L., Feder, M. Hubbard, W. N. , J. Phys . Chem. 5, 1337 (1962).

[2] Parker, V. B., Private communication, Nat. Bur. Std. (U.S.),

See Ref. [4] for a brief summary.

[3] Wagman, D. D. , Evans, W. H. , Parker, V. B., Halow, I., Bailey,

S. M., Schumm, R. H. , Nat. Bur. Std. (U.S.) Tech Note 270-4 (1969).

[4] Nuttall, R. L., Churney, K. L., Kilday, M. V., NBS Report 10904,

Chapt. 1, July 1 (1972).

[5] Wagman, D. D. , Evans, W. H. , Parker, V. B., Halow, I., Bailey, S. M.

,

Schumm, R. H. , Nat. Bur. Std. (U.S.) Tech Note 270-3 (1968).

[6] Wagman, D. D., Private communication, Nat. Bur. Std. (U.S.),

September^ 1973.

[7] O'Hare, P. A. G., Johnson, G. K. , Appelman, E. H. , J. Inorg.

Chem. 9, 332 (1970).

[8] Appelman, E. H. , and >klm, J. G., J. Am. Chem. Soc. 86, 2141 (1964).

[9] Parker, V. P., Thermal Properties of Aqueous Uni-valent Electrolytes^

National Standard Reference Data Series--NBS 2 U.S. Govt. Printing

Office, Washington, DC, 1965.

[10] Krause, R. F., NBS Report 10481, Chapt. 7, (1971).

[11] Prosen, E. J. and Kilday, M. V., J. Res. NBS 77A, 179 (1973).

85
6 . Appendix--Preliinlnary Experiments

A reaction scheme involving the reaction of MoF^(c) with an

oxidizing agent in aqueous base was investigated because it seemed

less likely^ a priori, that loss of reactants would occur in basic than

in acidic solutions due to the violence of the reaction. Both of tne two

oxidants considered, XeO^Caq) and H202(aq) in 0.3 NaOH, slowly decomposed

spontaneously when introduced into the calorimeter, as indicated by a

larger than normal drift rate (rate of change of the temperature of the

calorimeter with time). The decomposition of HXeO^ (the principal

species of XeO^ in our basic solution) before and after its reactions

with MoF^(c) occurred at constant but different rates, as inferred from

drift rate data.

The reaction scheme involving basic solutions of XeO^(aq) was

not pursued further since the decomposition implies the presence of

xenon in both the VI and VIII oxidation states and for the

change of xenon from the VIII to VI and VI to 0 oxidation states are

comparable and large in basic solution (see [8]). Thus, the relative

contributions of two oxidation reactions, written below in skeletal

form, would have to be evaluated and the heats of formation of both

Xe (VI) and Xe (VIII) in basic solution would have to be determined

MoF^(c) + [l/6IKeO^" + 6 S/eOH"] (goln) = MoO^ + 5F" + l/6Xe(g) + 7/2H2O] (soln)

(9)

MoF^(c) + [l/2HXeO^^" + 60H"] (soln) =

[MoO= + 5F" + l/2HXeO^" + 3H2O] (soln) (10)

86
It was of interest to find that our experimental observations in

basic solutions of XeO^(aq) are roughly consistent with those obtained

with HF solutions containing XeO^Caq) presented in the main text.

Assuming the rate of decomposition of HXeO^ during its reaction wi th

MoF^(c) was constant and equal to that after the reaction^ we obtained

a heat of reaction, due to both equations (9) and (10) of -738.9 kJ mol

^

-1 -1

(-176.6 kcal mol ) with an average deviation for five runs of 6.3 kJ mol

(1.5 kcal mol ^) . Assuming the heat of reaction of MoF^(^) with NaOH

is independent of the presence of XeO^(aq) and is equal to the average

value of -723 kJ mol""*" (172.9 kcal mol""*") obtained in our previous work

[4] on MoF^(^), we obtain for various mole fractions, X, of Xenon initially

present in the VIII oxidation state:

Zffl°[MoF, (J^)] - ZiI°[MoF^(c)] X

r b r J -197 kJ mol"''' (47.1 kcal mol"''") 0.0

-193 kJ mol"-"" (46a kcal mol'"'") 0.03

-183 kJ mol""'" (43.7 kcal mol"""") 0.10

These estimates neglect dilution corrections and are based on an

estimated enthalpy of formation of HXeO^ (aq) of +172 kJ mol "'" (+41 kcal mol

and ZXH°[HXeO^'^"(aq)] - AH° [HXeO^" (aq) ] + 418 kJ mol""*" (+100 kcal mol""'")".

In solutions containing Xe02(aq) and HF, drift rate data

indicated no decomposition of XeO^(aq) in the reactions involving

MoF^(c). In the case of the MoF^(i.) experiments, abnormally large

drift rates were observed when the monel holder for the sample bulbs

was present in the calorimeter as well as some reaction of the holder

with the solution, as indicated by a loss in its weight. Replacement

of the monel holder with a platinum holder in the final KoF.(Jl) experiments

b

eliminated these problems.

*The maximum uncertainty in AH° [MoF^ (-fc) ] -AH° [MoF^ (c) ] is -17 kJ mol

^

(4 kcal mol"-'-) .

87
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89
chapter 6

THE VAPOR PRESSURE AT 393 K OF LIQUID MOLYBDENUM PENTAFLUORIDE, MoF^,

ACCORDING TO PRELIMINARY STATIC-METHOD MEASUREMENTS

by Thomas B. Douglas and Ralph F. Krause, Jr.

Abstract

A new static -pressure method previously developed has

reproduced literature vapor-pressure values of I2 and MoF^ to

within about VL. In applying the method to MoF^ (X) in the presence

of M0F5 (g) to prevent dispropor tionation, it was found necessary

to crystallize the MoF^ to avoid solutions of unknown composition

of the components. Preliminary results on the saturated vapor of

M0F5 at 393 K indicate that it contains between 85 and 90 mole "L of

monomer

.

I . Introduction

In the two preceding annual technical reports of this series

[1, 2]''" we have reported our successful preparation, purification,

identification, characterization, stabilization, and transpiration

("entrainment") of high-purity molybdenum pentaf luoride, MoF^. Last

year's report [2] gave the results of completed transpiration measurements at three temperatures covering the range 343-383 K, and also

described in detail the design and preliminary testing of a new

Numbers in brackets refer to literature references at the end of

this chapter.

90
static-pressure method, introduced primarily to supplement the

transpiration data on MoF^ with equilibrium data of a different

type in order to gain quantitative information on the proportions

and thermodynamic properties of the different species (MoF^)^

known to be present in the vapor. After modifying the apparatus

and procedure to mimimize certain systematic errors, they met the

original goal of 1% accuracy by reproducing published accurate

vapor pressures of and MoFg with approximately this degree of

agreement. These successful tests with MoFg were made since the last

report, so the results are listed in the present report (Section II).

For the past several months the method has been applied to

2 measuring the total pressure of MoF^ vapor, and we had hoped to

have by this time a set of final results, at two or more temperatures,

on the saturated vapor and perhaps on an unsaturated state as well.

Although the v7ork is continuing, this stage has not yet been reached.

This is largely so because (as we discovered in our early transpiration

work on this substance ]1") a substantial proportion of MoF^ must be

added to prevent appreciable disproportionation of the MoF^, and

consequently the contribution of the MoF^ to the total pressure must

be accurately deducted. In addition, as is well known, static vapor

methods (unlike flow methods such as transpiration) deal with such

small masses of vapor that wall adsorption of some of it is not

negligible and must be accounted for in a reproducible way. Our

experience in handling these problems for MoF^ are discussed later

(Section III)

.

Although M0F5 vapor is a mixture of molecules (MoF5)j^ with different

values of n, it is often convenient to represent the vapor as a whole, or the crystal or liquid, by the simple formula MoF^

.

91
It was pointed out in last year's report [2] that while a

transpiration measurement producing a vapor composed of several

molecular species will in some cases give exactly the practical

information desired (the mass of material ablated by, or evaporated

into, a given flow of gas at a given temperature), neither the

apparent vapor pressure nor the apparent heat of vaporization, as

derived solely from these data, can be simply related to a single

vapor species or to a combination of them. Our early results for

the vapor pressure of MoF^, while imprecise, suggested that the

saturated vapor is highly associated and hence is likely to contain

several if not many species in major proportions (a conclusion we

later revised, as discussed in Section III). For this reason we

undertook, concurrently with further experimental work, a

mathematical exploration designed to answer several questions as to

how many different types of data (transpiration, saturated pressure,

unsaturated pressure, etc.) seem worthwhile, how accurate they need

to be, and how they can best be applied in interpreting the data

thermodynamically . Can one best use a classical thermodynamic

approach in which the vapor is simply considered a highly imperfect

gas, and if so, what minimum data are required? Or, if one elects

to recognize distinct molecular species in the vapor, how many

should be taken into account, and with what assumptions such that

the data errors are not too greatly magnified when specific-species

properties are derived? The significant results of this phase of

the investigation are discussed in another report [4].

92
Unlike substances of universal scientific or practical

importance, molybdenum pentaf luoride is not of such importance

by itself but that such a detailed study on it contrasts sharply

with many investigations which apply supposedly well-established

methods more or less routinely to a large list of different substances.

We feel, however, that much of the experimental and computative

information we are acquiring relative to MoF^ ni^y? rather than lie

buried in our notebooks, have equal value when applied to many

other substances which behave similarly. Some of the experimental

pitfalls of working with unstable substances have not been well

recognized, either by the experimenters or by those who review their

data. The same can be said for estimating the accuracy of derived

properties. If one succeeds in obtaining a reliable and comprehensive

set of results on one substance, the results may throw light on the

adequacy of competing methods applied to the same substance and

properties, as well as contribute to careful planning for future

work on other substances. And finally, so little reliable data exist

for the lower -valence fluorides of the transition elements that

information on one (MoF^) may contribute in the future to more

realistic estimated properties of other members of this class of

compounds

.

93
II. Tests of the New Apparatus by Measuring the Vapor Pressure of MoF,

6

In last year's report [2] we described in detail our development of a simple static method to measure small vapor pressures,

especially those of the order of 5 to 30 torr. In principle the

method is briefly as follows. A thermostated vessel of known volume

containing the vapor to be measured is momentarily opened to an

inert gas (argon) which had been at a slightly higher pressure,

measured by a sensitive manometer (di-n-butyl phthalate) . The known

volume ratio and the decrease in pressure of the external inert

gas determine the partial pressure of the portion of inert gas mixed

with the vapor, whose degree of association or dissociation has not

appreciably changed because its volume has remained constant. On

the assumption that total-pressure equilibrium has been established,

this partial pressure of inert gas may be subtracted from the final

total pressure to give the desired vapor pressure.

The most important requirement of the apparatus design and

procedure for this type of static-method approach is that virtual

total-pressure equilibrium be attained without appreciable vapor

diffusing out of its vessel and condensing or being absorbed. This

involves compromises, and that they are critical is evidenced by

the fact that we were able to eradicate major vapor-pressure errors

(consistently about 10%) only by making several modifications

described in the earlier report [2]. Thereafter, known pressures

of argon (simulating a vapor) were measured with a mean inaccuracy

94
of only 0.17c, and the published vapor pressures of iodine at

two temperatures were reproduced to about VL, but the measured

values for a vapor pressure of MoF, showed a spread of 9% [2].

o

After several apparatus and procedure improvements, 16

additional measurements of a vapor pressure of MoF^ (near 236 K,

approximately 10 torr) were made. These are individually compared

in Table 1 with the vapor-pressure value calculated from the

thermodynamic functions of Osborne e_t al_ [3], which should be

quite accurate; the disagreement averages about 0.7%. (Some of

these 16 measurements on MoF, were made not to test the method

6 but as auxiliary data needed in measuring MoF^ as described in

Section III .)

The most important improvements in technique preceding the

results of Table 1 are as follows. For the seat of the valve through

which the inert gas enters the vapor, Teflon replaces metal, making

leakage across the closed valve negligible. An Ascarite filter

was inserted between this valve and the manometer, thereby preventing

in successive valve openings an accummulation outside the vapor

vessel of MoF^ vapor that would react with the manometer oil and

affect future measurements appreciably. As in the earlier measurements [2], the valve between inert gas and vapor is still opened

usually twice to ensure pressure equilibration, but the optimum time

recipe applies to the first opening, viz., taking the accepted

manometer reading to 0.01 cm oil (0.008 torr) after opening the

95
Table 1. Test of the Static-Method: Vapor Pressure

of MoF^(s) at 236.60 K

(The pressure was observed in an isolated vessel at 298 K after

a period of equilibration with the solid sample near 236.60 K.

This observed vapor pressure was corrected to this nominal temperature, at which the value is 10.61 torr as calculated from the

thermochemical functions of Reference [3], The experiments are recorded in chronological order.)

Expt

.

No.

(P , /P , - 1)100 obs calc

7o

Expt

.

No.

(P , /P , - 1)100 obs calc

%

1 - 0.9 9 0.0

2 2.6 10 0.1

3 - 0.3 11 0.2

4 - 0.4 12 0.1

5 - 0.1 13 0.3

6 - 0.9 14 - 0.8

7

- 0.4 15 2.2

8 - 0.7 16 1.1

Standard deviation of the mean = 0.25%.

96
valve for 5 sec and then waiting no more than 1 min . The second

and successive valve openings lead to a drift of values, which

decrease by 0.02 to 0.03 torr for readings near 10 torr after waiting

5 min or more. This decrease is equivalent to an estimate of the

amount of MoF^ vapor which escapes the vapor vessel during each

valve opening and is adsorbed by the Ascarite.

The MoFg was purified as follows. A quantity was distilled

from its supply cylinder, condensed by Dry Ice, pumped for 15 min,

then allowed to warm to room temperature, and finally less than

half distilled to a trap thermostated at about 236 K. The vapor

from this source was then admitted to the previously evacuated vapor

vessel (A or C of Fig. 1, Section III) before beginning static-pressure

measurements an hour later.

Several precautions observed may be mentioned in terms of

their effects on the measured vapor-pressure values reflected in

Table 1. At 236 K the vapor pressure of MoF. varies about 107o per

6

degree, so that the temperature accuracy of the crystalline source

of vapor must be high; in fact, acceptance of the standard thermocouple

tables instead of thermocouple calibration would have made an error

of about 4% in the measured vapor pressure. (A later thermocouple

recalibration indicated a temperature correction equivalent to

0.1% in the MoF^ vapor pressure.) In a typical experiment measuring

a vapor pressure of about 10 torr, the coefficient of thermal expansion

of the manometric oil and argon is such that an error of 1 deg in

97
measuring the manometer temperature would itself cause an error

of about 0.5% in the measured vapor pressure. One refinement applied

was to use a gas-thermometer technique to determine the true average

temperature of the part of the inert-gas vessel passing from room

temperature to the hot bath containing the vapor vessel; when the

latter is at 120°C (the higher temperature used in the work of Section

III), it was found that a dependence on spot-temperature measurements

would have introduced an error of about 0 . 17o in the vapor pressure.

An error of similar magnitude would have resulted if the expansion

of the glass vessels from room temperature to 120°C had been ignored.

III. Preliminary: Static-Method Vapor-Pressure

Measurements on MoF^ (?) at 392.6 K

The test measurements on "known" cases outlined in the preceding

section satisfied us that our original goal of developing a static

method for measuring simple vapor pressures in the range of 5 to

30 torr to an accuracy of about VL had been met. We then proceeded

to apply the method to measuring vapor pressures of liquid MoF^

.

Besides the lack of known values for comparison, this substance

presents the complication of a tendency to disproportionate which,

our transpiration work [1, 2] had showed, can be satisfactorily

suppressed only by having present a considerable partial pressure

of MoF^ vapor which must be determined independently and subtracted

6

from the total pressure of the vapor mixture to give the equilibrium

pressure of the MoF^ vapor.

98
Our first series of measurements on MoF^ were made using

the earlier version of the apparatus [2] in the following way. A

desired amount of MoF^ (15 to 30 times enough to saturate the

vapor space at the final temperature) was transpired into the

"vapor vessel/' which was near room temperature (where the

vapor pressure of MoF^ is of the order of 0.01 torr) . Before

each experiment the vessel containing the supercooled MoF^

liquid was opened to vacuum (near 10 ^ torr at the pump) for over

an hour. The vessel and the supercooled MoF^ (liquid) sample

were subsequently thermostated at 311 K, and then supposedly

equilibrated with MoF^ vapor at a pressure (50 to 100% of that

expected for MoF^ vapor) which was controlled by and accurately

known [3] because of its contact with the purified crystal thermostated

at a suitable low temperature. The vapor vessel was then isolated

by dosing the valve, heated to and thermally equilibrated at the

final temperature (393 or 408 K) . From the subsequently determined

total pressure was subtracted the partial pressure at this temperature

of the number of moles of MoF^ vapor present at the lower temperature.

The precision of our earliest vapor-pressure values for MoF^

(9 experiments near, and corrected to, 393 K; and 4 experiments near,

and corrected to, 408 K) was distinctly disappointing. These values

showed, however, some indication ofconverging toward a value which,

compared with the transpiration value at the same temperature, indicated

an average degree of association near 2. This led to the tentative

conclusion that saturated MoF^ vapor (at these temperatures) must

99
be composed of major proportions of several species [several values

of n for (MoF^)^], and instigated the examination of the several

alternative ways of handling the thermodynamics of such a vapor that

are described in another report [4].

In attempting to explain the poor precision and to uncover

consistent errors that improved precision might hide, rough calculations

suggested that the following systematic errors in the procedure

thus far for MoF^ may have been too large to dismiss without

experimental tests and, if necessary, application of suitable corrections:

(a) The fluoride vapors may have been reacting chemically with

the hot Pyrex walls. (In subsequent tests in which the valve to

the vapor vessel at 393 K was opened at hourly intervals, the total

number of gaseous moles thereby registered remained so constant as

to indicate that the walls were either inert or had become so.)

(b) Wall adsorption of MoF^ should not be a problem, owing to

the excess of the liquid phase. But in heating the isolated vessel

containing MoF^ by almost 100 deg, the decrease in wall adsorption

of the MoF may be too large to ignore. (In four separate special

6

experiments a glass vessel at 311 K was filled with MoF^ vapor to a

pressure of 6.606 torr, then closed and heated to 393 K. In each

of the last three experiments the subsequently measured pressure

at 393 K exceeded 6 .606(393/311) by 1.0%. The surface-to-volume

ratio of the vessel was nominally 1.6 cm but believed to be

actually greater by a factor of 3 . A treatment using this information

100
and the Brunauer-Enmett-Teller adsorption equation indicated a

decrease in adsorption equivalent to 0.07 torr of MoF, at the

o higher temperature for all pressures from 1 to 10 torr, and this

small correction was applied in all future measurements.)

(c) The amount of MoF^ dissolving in the liquid MoF^ may have

been too large to ignore. (A simple ideal -solution estimation

indicated a solubility of the order of 0.001 mol fraction-- too small

to lower the partial pressure of MoF^ by an amount outside the

accuracy of the method; but enough to increase the pressure of MoF,

by as much as 23% when the supercooled MoF^ liquid was heated from

311 to 393 K.)

(d) The liquid MoF^ may have contained enough lower subfluoride

(such as MoF^) to destroy an appreciable fraction of the MoF^ in a

reversal of the disproportionation . (Even though the supercooled

MoF^ liquid was supposedly equilibrated in each experiment of the firs

series, this treatment may not have been effective owing to the lack

of stirring. Error (d) existed in the transpiration measurements on

MoF^ [1, 2] except that in that case there was no necessity of

accounting for the total MoF^ introduced, and the large excess used

was sufficient to purify increasingly the MoF^ liquid along its

length in the transpiration tube. However, in a static method an

impurity of only 0.15 mol % [1] in the MoF^ could under certain

circumstances lead to a fatal error. Error (d) is like Error (c)

above in being worse, but of opposite sign, for larger liquid

101
samples. Before we were aware of the importance of Error (d)^ we

had hoped to make Error (c) negligible and have a better account

of the MoFg. In a second series of experiments, we used a much

smaller liquid sample, 3 times enough to saturate the vapor space

at the final temperature of 393 K; and expanded at 298 K, in

contrast to the earlier equilibration at 311 K, a predetermined

quantity of MoF^ vapor into the vapor vessel containing the supercooled

MoF^ liquid. We seemed to find, however, that resorting to a much

smaller liquid sample tends to introduce even greater errors, among

them in this case the failure to equilibrate the MoF^ sample

beforehand with MoF,.)

6

To avoid Errors (c) and (d) we revised the procedure as applied

to MoF^ so as to be able to subtract from the total pressure not the

original but the final amount of MoF^, and the apparatus modified

o accordingly is diagrammed in Fig. 1. (The volume of each vessel A,

3 B, and C is about 80 cm .) Before the first subsequent run, the MoF^

sample (about 10 times the amount needed to fill Vessels A, B, and C

with saturated vapor at 393 K) was entrained into B while Vessels

A and C were detached. After installing, pretreating, and evacuating

A and C, vessel B at room temperature was opened to vacuum for about

an hour. (In this series a vacuum gauge which was installed near

the vessels served as an evacuation test of 10 ^ torr after the pump

was isolated.)

102
103
In contrast to the two earlier series, vacuum pumping was

applied at 298 K only to crystalline, not supercooled liquid, MoF^.

A suitable pressure of MoF^ was then equilibrated for about an

hour with the crystalline MoF^ at 298 K. As shown later, this

suitable pressure was several times smaller than a critical pressure

needed to support a MoF^ - containing liquid in equilibrium with

crystalline MoF^ . Valves a and d were then closed (but b and £

left open--they have no capillaries, in order to promote diffusion

of MoF^ vapor from B into A and C), and the bath was then heated

during the next hour to a nominal temperature of 393 K and

thermostated at that temperature for the next hour before beginning

static-pressure measurements.

After closing Valves b^ and £, the total pressure in A or C--

or, separately, in both--was then measured in the manner previously

described, by admitting argon through a_ or d_ respectively. The bath

was then cooled (45 min) to a nominal temperature of 298 K, and after

another hour the total pressure (now due almost entirely to MoF,)

D was measured by the standard procedure in A or C or, separately,

in both. When this measurement was on a vessel on which aneasurement had previously been made at the higher temperature, it was

necessary to subtract off the usually small contribution of the argon

that had previously entered before multiplying the observed MoFg

pressure by the exact temperature ratio (nominally, 393/298), adding

104
the adsorption correction previously discussed (0.07 torr), and

then subtracting this corrected value from the total pressure

measured at 393 to get the net vapor pressure of the MoF^ at

this temperature.

One further precaution observed in the above procedure should

be mentioned. On cooling A and C to 298 the formerly saturated

MoF^ vapor condenses and is so viscous that it normally supercools

and remains liquid, and a simple ideal -solution calculation suggests

that this liquid would be sufficient in amount to dissolve about

2% of the MoFg. However, the melting point of MoF^ is about 319 K

[1], and another ideal-solution calculation showed that at 298 K

pure crystalline MoF^ can exist in equilibrium with MoF^-containing

liquid only if the ambient pressure of MoF^ exceeds about 100 torr,

which is many times that in our work. Consequently, the crystallization

of the condensed MoF^ in A or C was always induced (by seeding with

2 a Dry-Ice slush applied externally over about 1 cm ) before undertaking

to measure the MoF^ pressure.

The essential results of all measurements made by the above

revised procedure up to the time of this writing are given in Table 2.

The so-called "experiments" (or runs) are numbered and enumerated

(one per line) in chronological order, as chronological trends appear

to be very significant. (The same charge of liquid MoF^ was used

for all eight experiments, and there is reason to suppose that the

purities of the sample portions in the three vessels changed

progressively, if at all, during the course of these experiments.) The

105
Table 2. Static-Method Measurements of the

Vapor Pressure of MoFi-(iO at 392.6 K

(The data of an experiment are used to calculate the vapor pressure

in as many as 3 alternate ways. All pressures are corrected to 392.6 K

as explained in the text, pc represents SPn[ (MoFr)^(g) ] in torr,

and pg represents P[MoFg(g)] in torr. 1 torr = 101,325/760 N/m2.)

1 2 3 4 5 6 7 8

Expt.

No.^

5 6 - P6 (obs)^ P5 (calc), using

:

(obs)^ initial

A and C

f inal^ initial

A and C

final

A C A C

1 7.74

d

1.70 1.74 6.04 6. 00

2 16.26 9.36 3.98 5.60d 6.90 12.28 10.66

3 19.77 9.42 7.57 10.35 12 .20

4 21.29 9.40 7.26 9. 15 11.89 14.03 12.14

5 21.27 9.27 7.96 9.29 12.00 13 .31 11.98

6 20.67 9.24 8.36d 9.51 11.43 12.31 11.16

7 20.73 9.26 8.76^ 9.55 11.47 11.97 11.18

8 12,87 1.41 11.46

Mean (Expt. 4-7) 11.70 12.90 11.62

Measured in vessel A for experiments 2, 4, 5, and 8 and in vessel C

for experiments 1, 3, 4, 5, 6, and 7. Differences of 0.2 and 0.3% were observed for experiments 4 and 5 respectively.

'^Measured same day as (p^ + pg) for experiments 6, 7, and 8, but the

next day for the other experiments.

Applied correction for decrease of wall adsorption (but no correction

for initial values, in which decrease of wall adsorption was assumed

to cancel increase of M0F5 solution in MoF^(£)).

*^No prior introduction of argon, which was corrected for in all other

final Pg values.

Not related to the Experiment Nos . of Table 1.

106
partial pressures of the individual vapor species are assumed

additive^ with p^ symbolizing the sum of the (MoF^)^ partial

pressures ("vapor pressure" of MoF^ (^0) ^^d p^ symbolizing the

pressure or partial pressure of MoF^, which is known not to

associate (except in the dispersion sense, which can be ignored

here). The higher temperature actually varied from 392.38 to

392.64 K, but for comparison purposes all the values have been

corrected to 392.6 K by assuming that p^ is proportional to the

absolute temperature, and that near 392.6 K, p^ increases by 5.8%

per deg [2]

.

The total pressure (p^ + p^) was measured sometimes in Vessel

A, sometimes in Vessel C, and in two experiments in both A and C

with only about 0.05 torr disagreement, so that a single value is

listed for each experiment. On the other hand we may consider that

we have for each (complete) experiment three generally different

values for p^ (Columns 3-5), which when subtracted from Column 2

give three alternate values for p^ (Columns 6-8, respectively)

.

(Column 3 was derived from the vapor pressure of MoF^ at its condensed

D source [3]; Columns 4 and 5 are as measured in the foregoing

procedure .)

Experiments 1 and 8, with no MoF^ deliberately introduced

(Column 3), were undertaken to allow an estimate of the residual

disproportionation in the other experiments, and will be discussed

later. Let us direct our attention first to Experiments 2-7. According

to the earlier discussion, the use of either Column 4 or 5 should

obviate error arising from failure to account for whatever MoF,

b

107
is absorbed by the bulk of liquid MoF^ (in Vessel B), and indeed

each of these two columns is seen to increase sharply with time

and in the later experiments approach the neighborhood of the

respective initial "pressures" of MoF^ shown in Column 3. Yet

the corresponding Columns 7 and 8 are not constant, but (except

for some unexplained irregularity at first) drift downward with

time, and may be approaching, at different rates but rather

asymptotically, approximately the same value. Although other

factors, such as possibly a more volatile impurity at first, may

be involved, we are inclined to feel that the downward drifts of

Columns 7 and 8 reflect persistent but declining losses of MoF^

vapor (different in A than in C), through reaction with the walls

or impurity in the condensed MoF^, that eventually would have become

completed.

In this connection it is significant that the vapor-pressure

values of Column 6, which were calculated in the earlier manner,

settle down after Experiment 3 to the neighborhood of the "asymptotic"

limits of Columns 7 and 8. This seems not surprising if we assume

that by the later experiments the bulk of liquid MoF^ has had its

postulated dispropor tionation reversed to the point where in each

succeeding experiment this liquid absorbs MoF^ at 393 K only through

solubility, an effect estimated to be perhaps 1 to 27o and offset by

a wall desorption (deliberately uncorrected for in Columns 3 and 6)

of similar magnitude but opposite sign.

108
In the light of the above discussion, Experiment 1 is premature

for estimating the disproportionation equilibrium, but Experiment

8 was used for this purpose. Assuming the disproportionation products

of MoF^ to be MoF^ and MoF^ (the latter is uncertain, but still lower

fluorides such as MoF^ would lead to even less serious error), the

1.41 torr of MoF^ measured in Experiment 8 determines approximately

the concentration of MoF^ dissolved in our particular amount of liquid

MoF^, and suggests for our case a mole fraction of about 0.01 for

the MoF^ in the liquid and hence a vapor-pressure of MoF^ (MoF^ is

much less volatile) too low by 17o owing to this cause. However, the

same equilibrium constant indicates that in Experiments 2-7, where

the pressure of MoF^ is about 5 times as great, the mol fraction of

MoF_^ would be only about 0.002 (0,27o error).

Finally, it is of Interest to examine what our data so far

indicate for the composition of saturated (MoF^)^ vapor at 392.6 K.

The two-constant "best" empirical equation fitting our transpiration

data at 343, 363, and 383 K [2] gives EnP^ = 13.3 torr at 392.6 K,

and we can tentatively take EP^ = 11.7 torr from Table 2. If we

assume only monomer and dimer, these two values give P-j^ = 10.1 torr,

P2 = 1.6 torr, and mole fractions of 0.86 and 0.14, respectively. On

the other hand, if we follow the treatment of Section II 2 of a related

chapter in another report [4] by assuming values of n from 1 to ^

but a constant ratio P , , /P , we get P, = 10.3 torr, P^ = 1.2 torr, n + 1 n^ ° 1 '2 '

P^ = 0.15 torr, and mole fractions of 0.88, 0.11, 0.015,

109
respectively (with correspondingly smaller proportions of the higher

species) . The two treatments thus indicate approximately the same

abundances of the monomer and the dimer.

We hope to have the opportunity to make some additional staticpressure measurements of the vapor pressure of MoF^ at 392.6 K in

order to test the validity of the trends appearing in Table 1, and

to extend the measurements to another temperature sufficiently different

as to enable us to determine the heats of the reactions involving

MoF^C-e), MoF^(g), and /or (MoF^)2(g).

IV . References

[1] R. F. Krause, Jr., AFOSR Scientific Report AFOSR-TR-71-2584

(NBS Report 10481), National Bureau of Standards, Washington,

D. C. 20234, July 1971, Chapter 7 (pp. 97-109).

[2] T. B. Douglas and R. F. Krause, Jr., AFOSR Scientific Report

AFOSR-TR-72-2004 (NBS Report 10904), National Bureau of Standards,

Washington, D. C. 20234, July 1972, Chapter 2 (pp. 29-48).

[3] D. W. Osborne, F. Schreiner, J. G. Malm, H. Selig, and L.

Rochester, J. Chem. Phys . 44, 2802-2809 (1966).

[4] T. B. Douglas and R. F. Krause, Jr., "Alternative Procedures

in Interpreting Equilibrium -Data on Partially Associated

Vapors," Chapter 4 of Report NBSIR 73-280, National Bureau of

Standards, Washington, D. C. 20234 (AFOSR-ISSA-72-0007) , 1

January 1973

.

110

USCOMM-NBS-DC
UNCUSSIFIED

JANUARY 2, 1974

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(Security ctmBaltlcation ol title, body of abstract and indeulng annotation umt be entered when the overall eport Im ctammltled)

1 ORIGINATING ACTIVITY (Corporate author) NATIONAL BUREAU OF STANDARDS

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Za. REfORT SECURITY CLASSIFICATION UNCLASSIFIED

26. CROUP

3 REPORT TITLE THERMODYNAMICS OF CHEMICAL SPECIES IMPORTANT TO ROCKET TECHNOLOGY

4 DESCRIPTIVE NOTES (Type ol rtpott and Inclualva dmtam) Scientific Final

S AUTHORISI (FIrmt name, middle Initial, tamt name) Thomas B. Douglas

Charles W. Beckett

e REPORT D-ATE 1 July 1973

la. TOTJ^L NO OF PACES 76. NO. OF REFS 120 56

Ba. CONTRACT OR GRANT NO AFOSR-ISSA-73-OOOl

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thim report) AFOSR-TR-74-0063

10 DISTRIBUTION STATEMENT

Approved for public release; distribution unlimited.

11 SUPPLEMENTARY NOTES

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12 SPONSO RING Ml LI T AR Y ACTIVITY AF Office of Scientific Research/NC

1400 Wilson Boulevard

Arlington, Virginia 22209

13 AB5TRAC T Using a subsecond -duration transient technique, the specific heat, electrical

resistivity, and hemispherical total emittance were simultaneously measured 1500-

2800 K for iron and the alloy 80Nb-10Ta-10W (estimated uncertainties: 3%, 0.5-1%,

and 37o for the respective properties) . Comparisons are made with generalized

approximations. At the gamma-to-delta transformation of iron (about 1680 K), the

temperature, heat of transformation, specific heat, spectral emittance, and

electrical resistivity were measured, thereby demonstrating the feasibility of

the technique for solid-solid transformations. Two other alloys were likewise

measured. Solution calorimetry involving several thermo chemical steps (including

oxidation by Xe03 in aqueous HF) gave the standard heat of formation of MoF5(c).

A new static vapor-pressure method, after verification to 1% by I2 and MoFg, and

after modifications to deal with the necessarily added MoFg, gave the vapor pressure

of M0F5 at 393 K and indicated (using also earlier NBS transpiration data) 85-90

mole % of monomer in the saturated vapor.

DD .Tv^t473 UNCLASSIFIED

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UNCLASSIFIED

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1 4 KEY WORDS

L 1 N K A L IN K B LINK C RO L t W T ROLE W T ROLE W T

ELECTRICAL RESISTIVITY

IRON

MOLYBDENUM PENTAFLUORIDE

PARTLY ASSOCIATED VAPORS

SOLID-STATE TRANSFORMATIONS

SOLUTION CALORIMETRY

SPECIFIC HEAT

SPECTRAL EMITTANCE

VAPOR PRESSURE ' - .

.

TRANSITION ALLOYS

UNCLASSIFIED

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